diff --git a/app/controllers/forms/welsh_translation_controller.rb b/app/controllers/forms/welsh_translation_controller.rb
index 83ce83430b..43f1bc15b9 100644
--- a/app/controllers/forms/welsh_translation_controller.rb
+++ b/app/controllers/forms/welsh_translation_controller.rb
@@ -77,8 +77,38 @@ def download
       form_content_service = WelshCsvService.new(form_with_pages_and_conditions)
 
       send_data form_content_service.as_csv,
-                type: "text/csv; charset=iso-8859-1",
-                disposition: "attachment; filename=#{form_content_service.filename}"
+                type: "text/csv; charset=utf-8",
+                filename: form_content_service.filename,
+                disposition: "attachment"
+    end
+
+    def show_upload
+      authorize current_form, :can_edit_form?
+      welsh_translation_upload_input = WelshTranslationUploadInput.new(form: current_form)
+      render :show_upload, locals: { current_form:, welsh_translation_upload_input: }
+    end
+
+    def upload
+      authorize current_form, :can_edit_form?
+
+      welsh_translation_upload_input = WelshTranslationUploadInput.new(**welsh_translation_upload_params)
+
+      data = welsh_translation_upload_input.read_file
+      unless data
+        return render :show_upload, status: :unprocessable_entity, locals: { current_form:, welsh_translation_upload_input: }
+      end
+
+      @welsh_translation_input = if FeatureService.new(group: current_form.group).enabled?(:multiple_branches)
+                                   WelshTranslationInput2.new(form: form_with_pages_and_exit_pages)
+                                 else
+                                   WelshTranslationInput.new(form: form_with_pages_and_conditions)
+                                 end
+
+      @welsh_translation_input.assign_from_spreadsheet(data).validate(:upload)
+      @table_presenter = Forms::TranslationTablePresenter.new
+
+      flash.now[:success] = t("banner.success.form.welsh_translation_uploaded")
+      render :new
     end
 
   private
@@ -122,5 +152,11 @@ def form_with_pages_and_conditions
     def form_with_pages_and_exit_pages
       Form.includes(pages: [:exit_pages]).find(current_form.id)
     end
+
+    def welsh_translation_upload_params
+      params.fetch(:forms_welsh_translation_upload_input, ActionController::Parameters.new)
+            .permit(:file)
+            .merge(form: current_form)
+    end
   end
 end
diff --git a/app/input_objects/forms/welsh_condition_translation_input.rb b/app/input_objects/forms/welsh_condition_translation_input.rb
index c7459b29c8..8b99897946 100644
--- a/app/input_objects/forms/welsh_condition_translation_input.rb
+++ b/app/input_objects/forms/welsh_condition_translation_input.rb
@@ -1,6 +1,7 @@
 class Forms::WelshConditionTranslationInput < BaseInput
   include ActionView::Helpers::FormTagHelper
   include ActiveModel::Attributes
+  include WelshTranslationContentLabels
 
   attr_accessor :condition
 
@@ -39,6 +40,17 @@ def assign_condition_values
     self
   end
 
+  def assign_from_spreadsheet(data)
+    assign_condition_values
+
+    %i[exit_page_heading exit_page_markdown].each do |attr|
+      content_label = condition_label(condition.routing_page, attr)
+      send(:"#{attr}_cy=", data[content_label]) if data.key?(content_label) && data[content_label].present?
+    end
+
+    self
+  end
+
   def form_field_id(attribute)
     field_id(:forms_welsh_condition_translation_input, condition.id, :condition_translations, attribute)
   end
diff --git a/app/input_objects/forms/welsh_exit_page_translation_input.rb b/app/input_objects/forms/welsh_exit_page_translation_input.rb
index c715fab37a..f5145cf59e 100644
--- a/app/input_objects/forms/welsh_exit_page_translation_input.rb
+++ b/app/input_objects/forms/welsh_exit_page_translation_input.rb
@@ -1,6 +1,7 @@
 class Forms::WelshExitPageTranslationInput < BaseInput
   include ActionView::Helpers::FormTagHelper
   include ActiveModel::Attributes
+  include WelshTranslationContentLabels
 
   attr_accessor :exit_page, :position
 
@@ -41,6 +42,17 @@ def assign_exit_page_values
     self
   end
 
+  def assign_from_spreadsheet(data)
+    assign_exit_page_values
+
+    %i[heading markdown].each do |attr|
+      content_label = exit_page_label(exit_page.question_page, position, attr)
+      send(:"#{attr}_cy=", data[content_label]) if data.key?(content_label) && data[content_label].present?
+    end
+
+    self
+  end
+
   def form_field_id(attribute)
     field_id(:forms_welsh_exit_page_translation_input, exit_page.id, :exit_page_translations, attribute)
   end
diff --git a/app/input_objects/forms/welsh_page_translation_input.rb b/app/input_objects/forms/welsh_page_translation_input.rb
index 20d90a655b..4310ebfbd2 100644
--- a/app/input_objects/forms/welsh_page_translation_input.rb
+++ b/app/input_objects/forms/welsh_page_translation_input.rb
@@ -2,6 +2,7 @@ class Forms::WelshPageTranslationInput < BaseInput
   include TextInputHelper
   include ActionView::Helpers::FormTagHelper
   include ActiveModel::Attributes
+  include WelshTranslationContentLabels
 
   attr_accessor :condition_translations, :selection_options_cy
   attr_reader :page
@@ -82,6 +83,27 @@ def assign_page_values
     self
   end
 
+  def assign_from_spreadsheet(data)
+    # assign values from the form first, and override only those that are set in the spreadsheet
+    assign_page_values
+
+    %i[question_text hint_text page_heading guidance_markdown none_of_the_above_question].each do |attr|
+      content_label = page_label(page, attr)
+      send(:"#{attr}_cy=", data[content_label]) if data.key?(content_label) && data[content_label].present?
+    end
+
+    selection_options_cy&.each_with_index do |option, index|
+      content_label = selection_option_label(page, index)
+      option.name_cy = data[content_label] if data.key?(content_label) && data[content_label].present?
+    end
+
+    condition_translations.each do |condition_translation|
+      condition_translation.assign_from_spreadsheet(data)
+    end
+
+    self
+  end
+
   def condition_translations_attributes=(attributes)
     submitted_condition_ids = attributes.values.map { |attrs| attrs["id"] }.compact
 
diff --git a/app/input_objects/forms/welsh_page_translation_input2.rb b/app/input_objects/forms/welsh_page_translation_input2.rb
index 84c2f2fa6b..aa6a62d4d7 100644
--- a/app/input_objects/forms/welsh_page_translation_input2.rb
+++ b/app/input_objects/forms/welsh_page_translation_input2.rb
@@ -2,6 +2,7 @@ class Forms::WelshPageTranslationInput2 < BaseInput
   include TextInputHelper
   include ActionView::Helpers::FormTagHelper
   include ActiveModel::Attributes
+  include WelshTranslationContentLabels
 
   attr_accessor :exit_page_translations, :selection_options_cy
   attr_reader :page
@@ -84,6 +85,27 @@ def assign_page_values
     self
   end
 
+  def assign_from_spreadsheet(data)
+    # assign values from the form first, and override only those that are set in the spreadsheet
+    assign_page_values
+
+    %i[question_text hint_text page_heading guidance_markdown none_of_the_above_question].each do |attr|
+      content_label = page_label(page, attr)
+      send(:"#{attr}_cy=", data[content_label]) if data.key?(content_label) && data[content_label].present?
+    end
+
+    selection_options_cy&.each_with_index do |option, index|
+      content_label = selection_option_label(page, index)
+      option.name_cy = data[content_label] if data.key?(content_label) && data[content_label].present?
+    end
+
+    exit_page_translations&.each do |exit_page_translation|
+      exit_page_translation.assign_from_spreadsheet(data)
+    end
+
+    self
+  end
+
   def exit_page_translations_attributes=(attributes)
     submitted_exit_page_ids = attributes.values.map { |attrs| attrs["id"] }.compact
 
diff --git a/app/input_objects/forms/welsh_selection_option_translation_input.rb b/app/input_objects/forms/welsh_selection_option_translation_input.rb
index e94025ac3d..00c9bad28f 100644
--- a/app/input_objects/forms/welsh_selection_option_translation_input.rb
+++ b/app/input_objects/forms/welsh_selection_option_translation_input.rb
@@ -26,6 +26,14 @@ def assign_selection_option_values
     self
   end
 
+  def assign_selection_option_values_from_csv_values(csv_values)
+    return self unless selection_option
+
+    spreadsheet_id = page_content_id(page.id, "option_#{index}")
+    self.name_cy = csv_values[spreadsheet_id] if csv_values.key?(spreadsheet_id)
+    self
+  end
+
   def as_selection_option
     { name: name_cy, value: selection_option.value }
   end
diff --git a/app/input_objects/forms/welsh_translation_input.rb b/app/input_objects/forms/welsh_translation_input.rb
index 7c0aca357b..2d5834a270 100644
--- a/app/input_objects/forms/welsh_translation_input.rb
+++ b/app/input_objects/forms/welsh_translation_input.rb
@@ -1,6 +1,7 @@
-class Forms::WelshTranslationInput < Forms::MarkCompleteInput
+class Forms::WelshTranslationInput < BaseInput
   include TextInputHelper
   include ActiveModel::Attributes
+  include WelshTranslationContentLabels
 
   attr_accessor :form, :page_translations
 
@@ -16,6 +17,10 @@ class Forms::WelshTranslationInput < Forms::MarkCompleteInput
   attribute :what_happens_next_markdown_cy
   attribute :payment_url_cy
 
+  with_options except_on: :upload do
+    validates :mark_complete, presence: true
+  end
+
   validates :name_cy, presence: true, if: -> { marked_complete? }
   validates :name_cy, length: { maximum: 500 }, if: -> { name_cy.present? }
 
@@ -122,6 +127,34 @@ def assign_form_values
     self
   end
 
+  def assign_from_spreadsheet(data)
+    # assign values from the form first, and override only those that are set in the spreadsheet
+    assign_form_values
+
+    %i[name
+       privacy_policy_url
+       support_email
+       support_phone
+       support_url
+       support_url_text
+       declaration_markdown
+       what_happens_next_markdown
+       payment_url].each do |attr|
+      content_label = FORM_ATTRIBUTE_LABELS.fetch(attr)
+      send(:"#{attr}_cy=", data[content_label]) if data.key?(content_label) && data[content_label].present?
+    end
+
+    self.page_translations = form.pages.map do |page|
+      Forms::WelshPageTranslationInput.new(page:).assign_from_spreadsheet(data)
+    end
+
+    self
+  end
+
+  def marked_complete?
+    ["true", true].include?(mark_complete)
+  end
+
   def blanked?
     all_fields_empty? && page_translations.all?(&:blanked?)
   end
diff --git a/app/input_objects/forms/welsh_translation_input2.rb b/app/input_objects/forms/welsh_translation_input2.rb
index 9a729aaa64..6be694438b 100644
--- a/app/input_objects/forms/welsh_translation_input2.rb
+++ b/app/input_objects/forms/welsh_translation_input2.rb
@@ -1,6 +1,7 @@
-class Forms::WelshTranslationInput2 < Forms::MarkCompleteInput
+class Forms::WelshTranslationInput2 < BaseInput
   include TextInputHelper
   include ActiveModel::Attributes
+  include WelshTranslationContentLabels
 
   attr_accessor :form, :page_translations
 
@@ -16,6 +17,10 @@ class Forms::WelshTranslationInput2 < Forms::MarkCompleteInput
   attribute :what_happens_next_markdown_cy
   attribute :payment_url_cy
 
+  with_options except_on: :upload do
+    validates :mark_complete, presence: true
+  end
+
   validates :name_cy, presence: true, if: -> { marked_complete? }
   validates :name_cy, length: { maximum: 500 }, if: -> { name_cy.present? }
 
@@ -122,6 +127,34 @@ def assign_form_values
     self
   end
 
+  def assign_from_spreadsheet(data)
+    # assign values from the form first, and override only those that are set in the spreadsheet
+    assign_form_values
+
+    %i[name
+       privacy_policy_url
+       support_email
+       support_phone
+       support_url
+       support_url_text
+       declaration_markdown
+       what_happens_next_markdown
+       payment_url].each do |attr|
+      content_label = FORM_ATTRIBUTE_LABELS.fetch(attr)
+      send(:"#{attr}_cy=", data[content_label]) if data.key?(content_label) && data[content_label].present?
+    end
+
+    self.page_translations = form.pages.map do |page|
+      Forms::WelshPageTranslationInput2.new(page:).assign_from_spreadsheet(data)
+    end
+
+    self
+  end
+
+  def marked_complete?
+    ["true", true].include?(mark_complete)
+  end
+
   def blanked?
     all_fields_empty? && page_translations.all?(&:blanked?)
   end
diff --git a/app/input_objects/forms/welsh_translation_upload_input.rb b/app/input_objects/forms/welsh_translation_upload_input.rb
new file mode 100644
index 0000000000..4242d3d3ae
--- /dev/null
+++ b/app/input_objects/forms/welsh_translation_upload_input.rb
@@ -0,0 +1,31 @@
+class Forms::WelshTranslationUploadInput < BaseInput
+  FILE_TYPES = %w[
+    text/csv
+  ].freeze
+  MAX_SIZE_IN_MB = 10
+
+  attr_accessor :form, :file
+
+  validates :file, presence: true, file_content_type: { in: FILE_TYPES }
+  validate :validate_file_size
+
+  def read_file
+    return false if invalid?
+
+    WelshCsvImportService.new(file, form).read
+  rescue CSV::MalformedCSVError
+    errors.add(:file, :malformed)
+    false
+  rescue WelshCsvImportService::InvalidHeadersError
+    errors.add(:file, :invalid_headers)
+    false
+  end
+
+private
+
+  def validate_file_size
+    if file.present? && file.size > MAX_SIZE_IN_MB.megabytes
+      errors.add(:file, :too_big)
+    end
+  end
+end
diff --git a/app/lib/welsh_translation_content_labels.rb b/app/lib/welsh_translation_content_labels.rb
new file mode 100644
index 0000000000..a889a11c0a
--- /dev/null
+++ b/app/lib/welsh_translation_content_labels.rb
@@ -0,0 +1,51 @@
+module WelshTranslationContentLabels
+  FORM_ATTRIBUTE_LABELS = {
+    name: "Form name",
+    declaration_markdown: "Declaration",
+    what_happens_next_markdown: "Information about what happens next",
+    payment_url: "GOV.UK Pay payment link",
+    privacy_policy_url: "Link to privacy information for this form",
+    support_email: "Contact details for support - email address",
+    support_phone: "Contact details for support - phone number and opening times",
+    support_url: "Contact details for support - online contact link",
+    support_url_text: "Contact details for support - online contact link text",
+  }.freeze
+
+  PAGE_ATTRIBUTE_LABELS = {
+    question_text: "question text",
+    hint_text: "hint text",
+    page_heading: "page heading",
+    guidance_markdown: "guidance text",
+    none_of_the_above_question: "question or label if 'None of the above' is selected",
+  }.freeze
+
+  CONDITION_ATTRIBUTE_LABELS = {
+    exit_page_heading: "exit page heading",
+    exit_page_markdown: "exit page content",
+  }.freeze
+
+  EXIT_PAGE_ATTRIBUTE_LABELS = {
+    heading: "heading",
+    markdown: "content",
+  }.freeze
+
+  def page_label(page, attribute)
+    "#{question_name(page)} - #{PAGE_ATTRIBUTE_LABELS.fetch(attribute)}"
+  end
+
+  def selection_option_label(page, option_index)
+    "#{question_name(page)} - option #{option_index + 1}"
+  end
+
+  def condition_label(page, attribute)
+    "#{question_name(page)} - #{CONDITION_ATTRIBUTE_LABELS.fetch(attribute)}"
+  end
+
+  def exit_page_label(page, exit_page_position, attribute)
+    "#{question_name(page)} - exit page #{exit_page_position} #{EXIT_PAGE_ATTRIBUTE_LABELS.fetch(attribute)}"
+  end
+
+  def question_name(page)
+    "Question #{page.position}"
+  end
+end
diff --git a/app/services/welsh_csv_import_service.rb b/app/services/welsh_csv_import_service.rb
new file mode 100644
index 0000000000..b7d75d854a
--- /dev/null
+++ b/app/services/welsh_csv_import_service.rb
@@ -0,0 +1,41 @@
+class WelshCsvImportService
+  include WelshTranslationContentLabels
+
+  class InvalidHeadersError < StandardError; end
+
+  attr_reader :file
+
+  HEADER_INDEXES = {
+    WelshCsvService::CONTENT_ID_HEADER => 0,
+    WelshCsvService::ENGLISH_CONTENT_HEADER => 1,
+    WelshCsvService::WELSH_CONTENT_HEADER => 2,
+  }.freeze
+
+  def initialize(file, form)
+    @file = file
+    @form = form
+  end
+
+  def read
+    file_content = file.read.force_encoding("UTF-8")
+    file_content.delete_prefix!("\xEF\xBB\xBF") # delete UTF-8 BOM if present
+    csv = CSV.parse(file_content, headers: true)
+
+    raise InvalidHeadersError unless headers_valid?(csv)
+
+    csv.each_with_object({}) do |row, values|
+      content_id = row[WelshCsvService::CONTENT_ID_HEADER]
+      next if content_id.nil?
+
+      values[content_id] = row[WelshCsvService::WELSH_CONTENT_HEADER].to_s
+    end
+  end
+
+private
+
+  def headers_valid?(csv)
+    HEADER_INDEXES.all? do |header, index|
+      csv.headers[index] == header
+    end
+  end
+end
diff --git a/app/services/welsh_csv_service.rb b/app/services/welsh_csv_service.rb
index addf53bce7..bb0822b2af 100644
--- a/app/services/welsh_csv_service.rb
+++ b/app/services/welsh_csv_service.rb
@@ -1,6 +1,13 @@
 class WelshCsvService
+  include WelshTranslationContentLabels
+
   MAX_FILENAME_LENGTH = 80
   FILENAME_SEPARATOR = "_".freeze
+  CONTENT_ID_HEADER = "Content ID".freeze
+  ENGLISH_CONTENT_HEADER = "English content".freeze
+  WELSH_CONTENT_HEADER = "Welsh content".freeze
+
+  UTF_8_BOM = "\uFEFF".freeze
 
   attr_reader :form
 
@@ -9,12 +16,15 @@ def initialize(form)
   end
 
   def as_csv
-    CSV.generate do |csv|
+    csv = CSV.generate do |csv|
       add_header(csv)
       add_form_name(csv)
       add_page_content(csv)
       add_form_metadata(csv)
     end
+
+    # Prepend UTF-8 BOM so Excel recognises the file as UTF-8 and preserves special characters
+    "#{UTF_8_BOM}#{csv}"
   end
 
   def filename
@@ -29,7 +39,7 @@ def filename
 private
 
   def add_header(csv)
-    csv << ["", "English content", "Welsh content"]
+    csv << [CONTENT_ID_HEADER, ENGLISH_CONTENT_HEADER, WELSH_CONTENT_HEADER]
   end
 
   def add_form_name(csv)
@@ -53,16 +63,16 @@ def add_page_content(csv)
   end
 
   def add_question_content(csv, page)
-    csv << ["#{question_name(page)} - question text", page.question_text, page.question_text_cy]
+    csv << [page_label(page, :question_text), page.question_text, page.question_text_cy]
     if page.hint_text.present?
-      csv << ["#{question_name(page)} - hint text", page.hint_text, page.hint_text_cy]
+      csv << [page_label(page, :hint_text), page.hint_text, page.hint_text_cy]
     end
   end
 
   def add_selection_options(csv, page)
     page.answer_settings.selection_options.each_with_index do |option, index|
       welsh_option_name = page.answer_settings_cy&.selection_options&.dig(index)&.name || ""
-      csv << ["#{question_name(page)} - option #{index + 1}", option.name, welsh_option_name]
+      csv << [selection_option_label(page, index), option.name, welsh_option_name]
     end
   end
 
@@ -71,7 +81,7 @@ def add_none_of_above_question(csv, page)
     welsh_question = page.answer_settings_cy&.none_of_the_above_question&.question_text || ""
 
     csv << [
-      "#{question_name(page)} - question or label if ‘None of the above’ is selected",
+      page_label(page, :none_of_the_above_question),
       english_question,
       welsh_question,
     ]
@@ -85,8 +95,8 @@ def has_none_of_the_above?(page)
   def add_routing_conditions(csv, page)
     page.routing_conditions.each do |condition|
       if condition.is_exit_page?
-        csv << ["#{question_name(page)} - exit page heading", condition.exit_page_heading, condition.exit_page_heading_cy]
-        csv << ["#{question_name(page)} - exit page content", condition.exit_page_markdown, condition.exit_page_markdown_cy]
+        csv << [condition_label(page, :exit_page_heading), condition.exit_page_heading, condition.exit_page_heading_cy]
+        csv << [condition_label(page, :exit_page_markdown), condition.exit_page_markdown, condition.exit_page_markdown_cy]
       end
     end
   end
@@ -95,20 +105,20 @@ def add_exit_pages(csv, page)
     exit_page_positions = ExitPage.positions_for_page(page)
     page.exit_pages.each do |exit_page|
       exit_page_position = exit_page_positions[exit_page.id]
-      csv << ["#{question_name(page)} - exit page #{exit_page_position} heading", exit_page.heading, exit_page.heading_cy]
-      csv << ["#{question_name(page)} - exit page #{exit_page_position} content", exit_page.markdown, exit_page.markdown_cy]
+      csv << [exit_page_label(page, exit_page_position, :heading), exit_page.heading, exit_page.heading_cy]
+      csv << [exit_page_label(page, exit_page_position, :markdown), exit_page.markdown, exit_page.markdown_cy]
     end
   end
 
   def add_page_heading(csv, page)
     if page.page_heading.present?
-      csv << ["#{question_name(page)} - page heading", page.page_heading, page.page_heading_cy]
+      csv << [page_label(page, :page_heading), page.page_heading, page.page_heading_cy]
     end
   end
 
   def add_guidance_text(csv, page)
     if page.guidance_markdown.present?
-      csv << ["#{question_name(page)} - guidance text", page.guidance_markdown, page.guidance_markdown_cy]
+      csv << [page_label(page, :guidance_markdown), page.guidance_markdown, page.guidance_markdown_cy]
     end
   end
 
@@ -117,19 +127,19 @@ def question_name(page)
   end
 
   def add_form_metadata(csv)
-    add_field_if_present(csv, "Declaration", form.declaration_text, form.declaration_text_cy)
-    add_field_if_present(csv, "Information about what happens next", form.what_happens_next_markdown, form.what_happens_next_markdown_cy)
-    add_field_if_present(csv, "GOV⁠.⁠UK Pay payment link", form.payment_url, form.payment_url_cy)
-    add_field_if_present(csv, "Link to privacy information for this form", form.privacy_policy_url, form.privacy_policy_url_cy)
+    add_field_if_present(csv, FORM_ATTRIBUTE_LABELS[:declaration_markdown], form.declaration_text, form.declaration_text_cy)
+    add_field_if_present(csv, FORM_ATTRIBUTE_LABELS[:what_happens_next_markdown], form.what_happens_next_markdown, form.what_happens_next_markdown_cy)
+    add_field_if_present(csv, FORM_ATTRIBUTE_LABELS[:payment_url], form.payment_url, form.payment_url_cy)
+    add_field_if_present(csv, FORM_ATTRIBUTE_LABELS[:privacy_policy_url], form.privacy_policy_url, form.privacy_policy_url_cy)
 
     add_support_details(csv)
   end
 
   def add_support_details(csv)
-    add_field_if_present(csv, "Contact details for support - email address", form.support_email, form.support_email_cy)
-    add_field_if_present(csv, "Contact details for support - phone number and opening times", form.support_phone, form.support_phone_cy)
-    add_field_if_present(csv, "Contact details for support - online contact link", form.support_url, form.support_url_cy)
-    add_field_if_present(csv, "Contact details for support - online contact link text", form.support_url_text, form.support_url_text_cy)
+    add_field_if_present(csv, FORM_ATTRIBUTE_LABELS[:support_email], form.support_email, form.support_email_cy)
+    add_field_if_present(csv, FORM_ATTRIBUTE_LABELS[:support_phone], form.support_phone, form.support_phone_cy)
+    add_field_if_present(csv, FORM_ATTRIBUTE_LABELS[:support_url], form.support_url, form.support_url_cy)
+    add_field_if_present(csv, FORM_ATTRIBUTE_LABELS[:support_url_text], form.support_url_text, form.support_url_text_cy)
   end
 
   def add_field_if_present(csv, label, english_value, welsh_value)
diff --git a/app/views/forms/welsh_translation/show_upload.html.erb b/app/views/forms/welsh_translation/show_upload.html.erb
new file mode 100644
index 0000000000..ef4950a310
--- /dev/null
+++ b/app/views/forms/welsh_translation/show_upload.html.erb
@@ -0,0 +1,32 @@
+<% set_page_title(t("page_titles.welsh_translation_upload")) %>
+<% content_for :back_link, govuk_back_link_to(welsh_translation_path(current_form)) %>
+
+<div class="govuk-grid-row">
+  <div class="govuk-grid-column-two-thirds">
+    <%= form_with(model: welsh_translation_upload_input, url: welsh_translation_upload_path(current_form), method: 'POST') do |f| %>
+      <% if welsh_translation_upload_input&.errors.any? %>
+        <%= f.govuk_error_summary %>
+      <% end %>
+
+      <h1 class="govuk-heading-l">
+        <span class="govuk-caption-l"><%= current_form.name %></span>
+        <%= t("page_titles.welsh_translation_upload") %>
+      </h1>
+
+      <%= govuk_warning_text(text: t(".warning")) %>
+
+      <%= govuk_details(summary_text: t(".details_summary")) do %>
+        <%= t(".details_html")%>
+      <% end %>
+
+      <%= f.govuk_file_field :file,
+        label: { tag: 'h2', size: 'm' },
+        accept: Forms::WelshTranslationUploadInput::FILE_TYPES.join(", "),
+        javascript: true
+      %>
+
+      <%= f.govuk_submit t("save_and_continue") %>
+    <% end %>
+  </div>
+</div>
+
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 4c27868a96..1a3853a53b 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -311,6 +311,7 @@ en:
         support_details_saved: Your contact details for support have been saved
         welsh_translation_saved: The Welsh version of your form has been saved
         welsh_translation_saved_and_completed: The Welsh version of your form has been saved and marked as complete
+        welsh_translation_uploaded: Your CSV has been uploaded
         what_happens_next_saved: Your information about what happens next has been saved
       page_routes_deleted: Question %{question_number}’s routes have been deleted
       route_created: Route %{route_number} has been added
@@ -751,6 +752,16 @@ en:
         support_url: Online contact link
         support_url_text: Text to describe the contact link
         welsh_header: Welsh content
+      show_upload:
+        details_html: |
+          <p>To make sure the formatting is correct:</p>
+          <ul class="govuk-list govuk-list--number">
+            <li>Download the existing content as a CSV.</li>
+            <li>Add or update Welsh content - without making any other changes to the formatting.</li>
+            <li>Save, then upload the new Welsh content as a CSV.</li>
+          </ul>
+        details_summary: How to format a CSV for uploading
+        warning: This will replace any Welsh content you already have saved in the form
   group_forms:
     edit:
       body_html: "<p> We’ll send an email to members of the current group to let them know the form has moved and they may no longer have access to it. </p>\n"
@@ -1839,6 +1850,7 @@ en:
     type_of_answer: What kind of answer do you need to this question?
     unarchive_form: Make your form live again
     welsh_translation: Add a Welsh version of your form
+    welsh_translation_upload: Upload new Welsh content as a CSV
     what_happens_next: Add information about what happens next
     your_changes_are_live: Your changes are live
     your_english_form_is_live: Your English form is live
diff --git a/config/locales/input_objects/welsh_translation_upload.yml b/config/locales/input_objects/welsh_translation_upload.yml
new file mode 100644
index 0000000000..97b0055125
--- /dev/null
+++ b/config/locales/input_objects/welsh_translation_upload.yml
@@ -0,0 +1,17 @@
+---
+en:
+  activemodel:
+    errors:
+      models:
+        forms/welsh_translation_upload_input:
+          attributes:
+            file:
+              blank: Select a file
+              invalid_file_type: The selected file must be a CSV
+              invalid_headers: We couldn’t upload the CSV because the column headings are wrong - check them and try uploading again
+              malformed: The CSV has invalid formatting - try downloading a new version, then uploading it again
+              too_big: The selected file must be smaller than 10MB
+  helpers:
+    label:
+      forms_welsh_translation_upload_input:
+        file: Upload CSV file
diff --git a/config/routes.rb b/config/routes.rb
index a604928485..8296d19653 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -81,6 +81,8 @@
     delete "/welsh-translation/delete" => "forms/welsh_translation#destroy", as: :welsh_translation_destroy
     post "/welsh-translation-preview" => "forms/welsh_translation#render_preview", as: :welsh_translation_render_preview
     get "/welsh-translation-download" => "forms/welsh_translation#download", as: :welsh_translation_download
+    get "/welsh-translation-upload" => "forms/welsh_translation#show_upload", as: :welsh_translation_show_upload
+    post "/welsh-translation-upload" => "forms/welsh_translation#upload", as: :welsh_translation_upload
     get "/submission-attachments" => "forms/submission_attachments#new", as: :submission_attachments
     post "/submission-attachments" => "forms/submission_attachments#create", as: :submission_attachments_create
     get "/batch-submissions" => "forms/batch_submissions#new", as: :batch_submissions
diff --git a/spec/fixtures/files/valid.csv b/spec/fixtures/files/valid.csv
new file mode 100644
index 0000000000..f39ee9076d
--- /dev/null
+++ b/spec/fixtures/files/valid.csv
@@ -0,0 +1,2 @@
+header1,header2
+value1,value2
\ No newline at end of file
diff --git a/spec/input_objects/forms/welsh_condition_translation_input_spec.rb b/spec/input_objects/forms/welsh_condition_translation_input_spec.rb
index 3b91a11fad..30e6ef4526 100644
--- a/spec/input_objects/forms/welsh_condition_translation_input_spec.rb
+++ b/spec/input_objects/forms/welsh_condition_translation_input_spec.rb
@@ -4,7 +4,7 @@
   subject(:welsh_condition_translation_input) { described_class.new(new_input_data) }
 
   let(:condition) { create_condition }
-  let(:page) { create :page }
+  let(:page) { create :page, position: 1 }
 
   let(:new_input_data) do
     {
@@ -16,12 +16,12 @@
 
   def create_condition(attributes = {})
     default_attributes = {
-      id: 1,
       answer_value: "Yes",
       exit_page_markdown: "You are ineligible",
       exit_page_heading: "Sorry, you are ineligible for this service.",
       exit_page_markdown_cy: "",
       exit_page_heading_cy: "",
+      routing_page: page,
     }
     create(:condition, default_attributes.merge(attributes))
   end
@@ -178,7 +178,7 @@ def create_condition(attributes = {})
     end
   end
 
-  describe "#assign_page_values" do
+  describe "#assign_condition_values" do
     it "loads the existing welsh attributes from the page" do
       welsh_condition_translation_input = described_class.new(condition:)
       welsh_condition_translation_input.assign_condition_values
@@ -188,6 +188,61 @@ def create_condition(attributes = {})
     end
   end
 
+  describe "#assign_from_spreadsheet" do
+    subject(:welsh_condition_translation_input) { described_class.new(condition:) }
+
+    context "when the spreadsheet data contains Welsh translations for all fields" do
+      let(:spreadsheet_data) do
+        {
+          "Question 1 - exit page heading" => "Welsh heading from spreadsheet",
+          "Question 1 - exit page content" => "Welsh markdown from spreadsheet",
+          "Question 2 - exit page heading" => "Another condition translation (ignored)",
+          "Question 1 - question text" => "Page field translation (ignored)",
+        }
+      end
+
+      it "assigns the welsh attributes from the spreadsheet data" do
+        welsh_condition_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_condition_translation_input.exit_page_heading_cy).to eq("Welsh heading from spreadsheet")
+        expect(welsh_condition_translation_input.exit_page_markdown_cy).to eq("Welsh markdown from spreadsheet")
+      end
+    end
+
+    context "when the spreadsheet data does not include keys for all fields" do
+      let(:condition) { create_condition(exit_page_markdown_cy: "Welsh markdown on form") }
+      let(:spreadsheet_data) do
+        {
+          "Question 1 - exit page heading" => "Welsh heading from spreadsheet",
+        }
+      end
+
+      it "uses the Welsh already set on the condition for fields not present in the spreadsheet data" do
+        welsh_condition_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_condition_translation_input.exit_page_heading_cy).to eq("Welsh heading from spreadsheet")
+        expect(welsh_condition_translation_input.exit_page_markdown_cy).to eq("Welsh markdown on form")
+      end
+    end
+
+    context "when the spreadsheet data includes blank values" do
+      let(:condition) { create_condition(exit_page_markdown_cy: "Welsh markdown on form", exit_page_heading_cy: "Welsh heading on form") }
+      let(:spreadsheet_data) do
+        {
+          "Question 1 - exit page heading" => "Welsh heading from spreadsheet",
+          "Question 1 - exit page content" => "",
+        }
+      end
+
+      it "uses the Welsh already set on the condition for fields with blank values in the spreadsheet data" do
+        welsh_condition_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_condition_translation_input.exit_page_heading_cy).to eq("Welsh heading from spreadsheet")
+        expect(welsh_condition_translation_input.exit_page_markdown_cy).to eq("Welsh markdown on form")
+      end
+    end
+  end
+
   describe "#condition_has_exit_page?" do
     context "when the condition has an exit page" do
       let(:condition) do
diff --git a/spec/input_objects/forms/welsh_exit_page_translation_input_spec.rb b/spec/input_objects/forms/welsh_exit_page_translation_input_spec.rb
index 44a1f36e41..9184a7f8e8 100644
--- a/spec/input_objects/forms/welsh_exit_page_translation_input_spec.rb
+++ b/spec/input_objects/forms/welsh_exit_page_translation_input_spec.rb
@@ -4,7 +4,7 @@
   subject(:welsh_exit_page_translation_input) { described_class.new(new_input_data) }
 
   let(:exit_page) { create_exit_page }
-  let(:page) { create :page }
+  let(:page) { create :page, position: 3 }
 
   let(:new_input_data) do
     {
@@ -17,7 +17,6 @@
 
   def create_exit_page(attributes = {})
     default_attributes = {
-      id: 1,
       question_page: page,
       markdown: "You are ineligible",
       heading: "Sorry, you are ineligible for this service.",
@@ -36,7 +35,7 @@ def create_exit_page(attributes = {})
 
         it "is not valid" do
           expect(welsh_exit_page_translation_input).not_to be_valid(validation_context)
-          expect(welsh_exit_page_translation_input.errors.full_messages_for(:heading_cy)).to include "Heading cy #{I18n.t('activemodel.errors.models.forms/welsh_condition_translation_input.attributes.exit_page_heading_cy.blank', question_number: exit_page.question_page.position)}"
+          expect(welsh_exit_page_translation_input.errors.full_messages_for(:heading_cy)).to include "Heading cy #{I18n.t('activemodel.errors.models.forms/welsh_exit_page_translation_input.attributes.heading_cy.blank', question_number: exit_page.question_page.position)}"
         end
       end
 
@@ -46,7 +45,7 @@ def create_exit_page(attributes = {})
 
           it "is not valid" do
             expect(welsh_exit_page_translation_input).not_to be_valid(validation_context)
-            expect(welsh_exit_page_translation_input.errors.full_messages_for(:heading_cy)).to include "Heading cy #{I18n.t('activemodel.errors.models.forms/welsh_condition_translation_input.attributes.exit_page_heading_cy.too_long', question_number: exit_page.question_page.position, count: 250)}"
+            expect(welsh_exit_page_translation_input.errors.full_messages_for(:heading_cy)).to include "Heading cy #{I18n.t('activemodel.errors.models.forms/welsh_exit_page_translation_input.attributes.heading_cy.too_long', question_number: exit_page.question_page.position, count: 250)}"
           end
         end
 
@@ -65,7 +64,7 @@ def create_exit_page(attributes = {})
 
         it "is not valid" do
           expect(welsh_exit_page_translation_input).not_to be_valid(validation_context)
-          expect(welsh_exit_page_translation_input.errors.full_messages_for(:markdown_cy)).to include "Markdown cy #{I18n.t('activemodel.errors.models.forms/welsh_condition_translation_input.attributes.exit_page_markdown_cy.blank', question_number: exit_page.question_page.position)}"
+          expect(welsh_exit_page_translation_input.errors.full_messages_for(:markdown_cy)).to include "Markdown cy #{I18n.t('activemodel.errors.models.forms/welsh_exit_page_translation_input.attributes.markdown_cy.blank', question_number: exit_page.question_page.position)}"
         end
       end
 
@@ -95,7 +94,7 @@ def create_exit_page(attributes = {})
 
           it "is not valid" do
             expect(welsh_exit_page_translation_input).not_to be_valid(validation_context)
-            expect(welsh_exit_page_translation_input.errors.full_messages_for(:heading_cy)).to include "Heading cy #{I18n.t('activemodel.errors.models.forms/welsh_condition_translation_input.attributes.exit_page_heading_cy.too_long', question_number: exit_page.question_page.position, count: 250)}"
+            expect(welsh_exit_page_translation_input.errors.full_messages_for(:heading_cy)).to include "Heading cy #{I18n.t('activemodel.errors.models.forms/welsh_exit_page_translation_input.attributes.heading_cy.too_long', question_number: exit_page.question_page.position, count: 250)}"
           end
         end
 
@@ -157,6 +156,62 @@ def create_exit_page(attributes = {})
     end
   end
 
+  describe "#assign_from_spreadsheet" do
+    subject(:welsh_exit_page_translation_input) { described_class.new(exit_page: exit_page, position: 1) }
+
+    context "when the spreadsheet data contains Welsh translations for all fields" do
+      let(:spreadsheet_data) do
+        {
+          "Question 3 - exit page 1 heading" => "Welsh heading from spreadsheet",
+          "Question 3 - exit page 1 content" => "Welsh markdown from spreadsheet",
+          "Question 3 - exit page 2 heading" => "Another exit page heading translation (ignored)",
+          "Question 2 - exit page 1 heading" => "Another page's exit page heading translation (ignored)",
+          "Question 1 - question text" => "Page field translation (ignored)",
+        }
+      end
+
+      it "assigns the welsh attributes from the spreadsheet data" do
+        welsh_exit_page_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_exit_page_translation_input.heading_cy).to eq("Welsh heading from spreadsheet")
+        expect(welsh_exit_page_translation_input.markdown_cy).to eq("Welsh markdown from spreadsheet")
+      end
+    end
+
+    context "when the spreadsheet data does not include keys for all fields" do
+      let(:exit_page) { create_exit_page(markdown_cy: "Welsh markdown on form") }
+      let(:spreadsheet_data) do
+        {
+          "Question 3 - exit page 1 heading" => "Welsh heading from spreadsheet",
+        }
+      end
+
+      it "uses the Welsh already set on the exit page for fields not present in the spreadsheet data" do
+        welsh_exit_page_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_exit_page_translation_input.heading_cy).to eq("Welsh heading from spreadsheet")
+        expect(welsh_exit_page_translation_input.markdown_cy).to eq("Welsh markdown on form")
+      end
+    end
+
+    context "when the spreadsheet data includes blank values" do
+      let(:exit_page) { create_exit_page(markdown_cy: "Welsh markdown on form", heading_cy: "Welsh heading on form") }
+      let(:spreadsheet_data) do
+        {
+          "Question 3 - exit page 1 heading" => "Welsh heading from spreadsheet",
+          "Question 3 - exit page 1 content" => "",
+        }
+      end
+
+      it "uses the Welsh already set on the exit page for fields with blank values in the spreadsheet data" do
+        welsh_exit_page_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_exit_page_translation_input.heading_cy).to eq("Welsh heading from spreadsheet")
+        expect(welsh_exit_page_translation_input.markdown_cy).to eq("Welsh markdown on form")
+      end
+    end
+  end
+
   describe "#form_field_id" do
     let(:exit_page) do
       create_exit_page(id: 999)
@@ -175,7 +230,7 @@ def create_exit_page(attributes = {})
       end
     end
 
-    context "when the welsh condition fields are all empty" do
+    context "when the welsh exit page fields are all empty" do
       let(:new_input_data) { { exit_page:, markdown_cy: "", heading_cy: "" } }
 
       it "returns true" do
diff --git a/spec/input_objects/forms/welsh_page_translation_input2_spec.rb b/spec/input_objects/forms/welsh_page_translation_input2_spec.rb
index a661034cde..6882e3ecea 100644
--- a/spec/input_objects/forms/welsh_page_translation_input2_spec.rb
+++ b/spec/input_objects/forms/welsh_page_translation_input2_spec.rb
@@ -585,6 +585,101 @@ def create_page(attributes = {})
     end
   end
 
+  describe "#assign_from_spreadsheet" do
+    let(:page) do
+      create_page(attributes_for(:page, :selection_with_none_of_the_above_question, position: 3))
+    end
+
+    before do
+      page.exit_pages = [exit_page, another_exit_page]
+    end
+
+    context "when the spreadsheet data contains Welsh translations for all fields" do
+      let(:spreadsheet_data) do
+        {
+          "Question 3 - question text" => "Welsh question text from spreadsheet",
+          "Question 3 - hint text" => "Welsh hint text from spreadsheet",
+          "Question 3 - page heading" => "Welsh page heading from spreadsheet",
+          "Question 3 - guidance text" => "Welsh guidance markdown from spreadsheet",
+          "Question 3 - option 1" => "Welsh Option 1 from spreadsheet",
+          "Question 3 - option 2" => "Welsh Option 2 from spreadsheet",
+          "Question 3 - question or label if 'None of the above' is selected" => "Welsh None of the above question? from spreadsheet",
+          "Question 3 - exit page 1 heading" => "Welsh exit page heading from spreadsheet",
+          "Question 3 - exit page 1 content" => "Welsh exit page markdown from spreadsheet",
+          "Question 3 - exit page 2 heading" => "Another exit page heading from spreadsheet",
+          "Question 3 - exit page 2 content" => "Another exit page markdown from spreadsheet",
+          "Question 1 - page heading" => "Translation for a different page (ignored)",
+          "Declaration" => "Translation for a form field (ignored)",
+        }
+      end
+
+      it "sets the welsh attributes from the spreadsheet data" do
+        welsh_page_translation_input = described_class.new(page:)
+        welsh_page_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_page_translation_input.question_text_cy).to eq("Welsh question text from spreadsheet")
+        expect(welsh_page_translation_input.hint_text_cy).to eq("Welsh hint text from spreadsheet")
+        expect(welsh_page_translation_input.page_heading_cy).to eq("Welsh page heading from spreadsheet")
+        expect(welsh_page_translation_input.guidance_markdown_cy).to eq("Welsh guidance markdown from spreadsheet")
+        expect(welsh_page_translation_input.none_of_the_above_question_cy).to eq("Welsh None of the above question? from spreadsheet")
+
+        selection_options_cy = welsh_page_translation_input.selection_options_cy.map(&:as_selection_option)
+
+        expect(selection_options_cy).to eq([
+          { name: "Welsh Option 1 from spreadsheet", value: "Option 1" },
+          { name: "Welsh Option 2 from spreadsheet", value: "Option 2" },
+        ])
+
+        exit_page_translation = welsh_page_translation_input.exit_page_translations.find { |ct| ct.id == exit_page.id }
+        expect(exit_page_translation.heading_cy).to eq("Welsh exit page heading from spreadsheet")
+        expect(exit_page_translation.markdown_cy).to eq("Welsh exit page markdown from spreadsheet")
+
+        another_exit_page_translation = welsh_page_translation_input.exit_page_translations.find { |ct| ct.id == another_exit_page.id }
+        expect(another_exit_page_translation.heading_cy).to eq("Another exit page heading from spreadsheet")
+        expect(another_exit_page_translation.markdown_cy).to eq("Another exit page markdown from spreadsheet")
+      end
+    end
+
+    context "when the spreadsheet data does not include keys for all fields" do
+      let(:page) { create_page(hint_text_cy: "Page Welsh hint text", position: 3) }
+      let(:spreadsheet_data) do
+        {
+          "Question 3 - question text" => "Welsh question text from spreadsheet",
+        }
+      end
+
+      it "uses the Welsh already set on the page for fields not present in the spreadsheet data" do
+        welsh_page_translation_input = described_class.new(page:)
+        welsh_page_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_page_translation_input.question_text_cy).to eq("Welsh question text from spreadsheet")
+        expect(welsh_page_translation_input.hint_text_cy).to eq("Page Welsh hint text")
+      end
+    end
+
+    context "when the spreadsheet data includes blank values" do
+      let(:page) do
+        create_page(question_text_cy: "Page Welsh question text",
+                    hint_text_cy: "Page Welsh hint text",
+                    position: 3)
+      end
+      let(:spreadsheet_data) do
+        {
+          "Question 3 - question text" => "Welsh question text from spreadsheet",
+          "Question 3 - hint text" => "",
+        }
+      end
+
+      it "uses the Welsh already set on the page for fields with blank values in the spreadsheet data" do
+        welsh_page_translation_input = described_class.new(page:)
+        welsh_page_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_page_translation_input.question_text_cy).to eq("Welsh question text from spreadsheet")
+        expect(welsh_page_translation_input.hint_text_cy).to eq("Page Welsh hint text")
+      end
+    end
+  end
+
   describe "#page_has_hint_text?" do
     context "when the page has hint_text" do
       let(:page) { create_page(hint_text: "Choose 'Yes' if you already have a valid licence.") }
diff --git a/spec/input_objects/forms/welsh_page_translation_input_spec.rb b/spec/input_objects/forms/welsh_page_translation_input_spec.rb
index eaf59b06d5..48e5fb4b5f 100644
--- a/spec/input_objects/forms/welsh_page_translation_input_spec.rb
+++ b/spec/input_objects/forms/welsh_page_translation_input_spec.rb
@@ -25,7 +25,6 @@
 
   def create_page(attributes = {})
     default_attributes = {
-      id: 1,
       question_text: "Are you renewing a licence?",
       hint_text: "Choose 'Yes' if you already have a valid licence.",
       page_heading: "Licencing",
@@ -541,8 +540,8 @@ def create_page(attributes = {})
 
         expect(selection_options_cy).to eq([
           { name: "Welsh option 1", value: "Yes" },
-          { name:  "", value: "No" },
-          { name:  "", value: "Maybe" },
+          { name: "", value: "No" },
+          { name: "", value: "Maybe" },
         ])
       end
     end
@@ -581,6 +580,95 @@ def create_page(attributes = {})
     end
   end
 
+  describe "#assign_from_spreadsheet" do
+    let(:page) do
+      create_page(attributes_for(:page, :selection_with_none_of_the_above_question, position: 3))
+    end
+
+    before do
+      page.routing_conditions = [condition, another_condition]
+    end
+
+    context "when the spreadsheet data contains Welsh translations for all fields" do
+      let(:spreadsheet_data) do
+        {
+          "Question 3 - question text" => "Welsh question text from spreadsheet",
+          "Question 3 - hint text" => "Welsh hint text from spreadsheet",
+          "Question 3 - page heading" => "Welsh page heading from spreadsheet",
+          "Question 3 - guidance text" => "Welsh guidance markdown from spreadsheet",
+          "Question 3 - option 1" => "Welsh Option 1 from spreadsheet",
+          "Question 3 - option 2" => "Welsh Option 2 from spreadsheet",
+          "Question 3 - question or label if 'None of the above' is selected" => "Welsh None of the above question? from spreadsheet",
+          "Question 3 - exit page heading" => "Welsh exit page heading from spreadsheet",
+          "Question 3 - exit page content" => "Welsh exit page markdown from spreadsheet",
+          "Question 1 - page heading" => "Translation for a different page (ignored)",
+          "Declaration" => "Translation for a form field (ignored)",
+        }
+      end
+
+      it "sets the welsh attributes from the spreadsheet data" do
+        welsh_page_translation_input = described_class.new(page:)
+        welsh_page_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_page_translation_input.question_text_cy).to eq("Welsh question text from spreadsheet")
+        expect(welsh_page_translation_input.hint_text_cy).to eq("Welsh hint text from spreadsheet")
+        expect(welsh_page_translation_input.page_heading_cy).to eq("Welsh page heading from spreadsheet")
+        expect(welsh_page_translation_input.guidance_markdown_cy).to eq("Welsh guidance markdown from spreadsheet")
+        expect(welsh_page_translation_input.none_of_the_above_question_cy).to eq("Welsh None of the above question? from spreadsheet")
+
+        selection_options_cy = welsh_page_translation_input.selection_options_cy.map(&:as_selection_option)
+
+        expect(selection_options_cy).to eq([
+          { name: "Welsh Option 1 from spreadsheet", value: "Option 1" },
+          { name: "Welsh Option 2 from spreadsheet", value: "Option 2" },
+        ])
+
+        condition_translation = welsh_page_translation_input.condition_translations.find { |ct| ct.id == condition.id }
+        expect(condition_translation.exit_page_heading_cy).to eq("Welsh exit page heading from spreadsheet")
+        expect(condition_translation.exit_page_markdown_cy).to eq("Welsh exit page markdown from spreadsheet")
+      end
+    end
+
+    context "when the spreadsheet data does not include keys for all fields" do
+      let(:page) { create_page(hint_text_cy: "Page Welsh hint text", position: 3) }
+      let(:spreadsheet_data) do
+        {
+          "Question 3 - question text" => "Welsh question text from spreadsheet",
+        }
+      end
+
+      it "uses the Welsh already set on the page for fields not present in the spreadsheet data" do
+        welsh_page_translation_input = described_class.new(page:)
+        welsh_page_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_page_translation_input.question_text_cy).to eq("Welsh question text from spreadsheet")
+        expect(welsh_page_translation_input.hint_text_cy).to eq("Page Welsh hint text")
+      end
+    end
+
+    context "when the spreadsheet data includes blank values" do
+      let(:page) do
+        create_page(question_text_cy: "Page Welsh question text",
+                    hint_text_cy: "Page Welsh hint text",
+                    position: 3)
+      end
+      let(:spreadsheet_data) do
+        {
+          "Question 3 - question text" => "Welsh question text from spreadsheet",
+          "Question 3 - hint text" => "",
+        }
+      end
+
+      it "uses the Welsh already set on the page for fields with blank values in the spreadsheet data" do
+        welsh_page_translation_input = described_class.new(page:)
+        welsh_page_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_page_translation_input.question_text_cy).to eq("Welsh question text from spreadsheet")
+        expect(welsh_page_translation_input.hint_text_cy).to eq("Page Welsh hint text")
+      end
+    end
+  end
+
   describe "#page_has_hint_text?" do
     context "when the page has hint_text" do
       let(:page) { create_page(hint_text: "Choose 'Yes' if you already have a valid licence.") }
diff --git a/spec/input_objects/forms/welsh_translation_input2_spec.rb b/spec/input_objects/forms/welsh_translation_input2_spec.rb
index 11ead9e6ef..b45d9dcbf8 100644
--- a/spec/input_objects/forms/welsh_translation_input2_spec.rb
+++ b/spec/input_objects/forms/welsh_translation_input2_spec.rb
@@ -12,7 +12,7 @@
            guidance_markdown: "This part of the form concerns licencing.",
            position: 1
   end
-  let(:another_page) { create :page }
+  let(:another_page) { create :page, position: 2 }
 
   let(:mark_complete) { "true" }
 
@@ -692,6 +692,91 @@ def build_empty_welsh_form
     end
   end
 
+  describe "#assign_from_spreadsheet" do
+    before do
+      form.pages = [page, another_page]
+    end
+
+    context "when the spreadsheet data contains Welsh translations for all fields" do
+      let(:spreadsheet_data) do
+        {
+          "Form name" => "Welsh A form from spreadsheet",
+          "Question 1 - question text" => "Welsh question text from spreadsheet",
+          "Question 1 - option 1" => "Welsh Option 1 from spreadsheet",
+          "Question 1 - option 2" => "Welsh Option 2 from spreadsheet",
+          "Question 1 - question or label if 'None of the above' is selected" => "Welsh None of the above question? from spreadsheet",
+          "Question 1 - exit page 1 heading" => "Welsh exit page heading from spreadsheet",
+          "Question 1 - exit page 1 content" => "Welsh exit page content from spreadsheet",
+          "Question 2 - page heading" => "Welsh question 2 page heading from spreadsheet",
+          "Question 2 - guidance text" => "Welsh question 2 guidance markdown from spreadsheet",
+          "Question 2 - question text" => "Welsh question 2 text from spreadsheet",
+          "Declaration" => "Welsh declaration text from spreadsheet",
+          "Information about what happens next" => "Welsh what happens next from spreadsheet",
+          "GOV.UK Pay payment link" => "https://www.gov.uk/payment_cy_spreadsheet",
+          "Link to privacy information for this form" => "https://www.gov.uk/privacy_cy_spreadsheet",
+          "Contact details for support - email address" => "support-spreadsheet@example.gov.wales",
+          "Contact details for support - phone number and opening times" => "Welsh support phone from spreadsheet",
+          "Contact details for support - online contact link" => "https://www.gov.uk/support_cy_spreadsheet",
+          "Contact details for support - online contact link text" => "Welsh Support URL text from spreadsheet",
+        }
+      end
+
+      it "sets the welsh attributes from the spreadsheet data" do
+        welsh_translation_input = described_class.new(form:)
+        welsh_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_translation_input.name_cy).to eq("Welsh A form from spreadsheet")
+        expect(welsh_translation_input.declaration_markdown_cy).to eq("Welsh declaration text from spreadsheet")
+        expect(welsh_translation_input.what_happens_next_markdown_cy).to eq("Welsh what happens next from spreadsheet")
+        expect(welsh_translation_input.payment_url_cy).to eq("https://www.gov.uk/payment_cy_spreadsheet")
+        expect(welsh_translation_input.privacy_policy_url_cy).to eq("https://www.gov.uk/privacy_cy_spreadsheet")
+        expect(welsh_translation_input.support_email_cy).to eq("support-spreadsheet@example.gov.wales")
+        expect(welsh_translation_input.support_phone_cy).to eq("Welsh support phone from spreadsheet")
+        expect(welsh_translation_input.support_url_cy).to eq("https://www.gov.uk/support_cy_spreadsheet")
+        expect(welsh_translation_input.support_url_text_cy).to eq("Welsh Support URL text from spreadsheet")
+
+        page_translation = welsh_translation_input.page_translations.find { |pt| pt.id == page.id }
+        expect(page_translation.question_text_cy).to eq("Welsh question text from spreadsheet")
+
+        another_page_translation = welsh_translation_input.page_translations.find { |pt| pt.id == another_page.id }
+        expect(another_page_translation.page_heading_cy).to eq("Welsh question 2 page heading from spreadsheet")
+      end
+    end
+
+    context "when the spreadsheet data does not include keys for all fields" do
+      let(:spreadsheet_data) do
+        {
+          "Form name" => "Welsh A form from spreadsheet",
+        }
+      end
+
+      it "uses the Welsh already set on the form for fields not present in the spreadsheet data" do
+        welsh_translation_input = described_class.new(form:)
+        welsh_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_translation_input.name_cy).to eq("Welsh A form from spreadsheet")
+        expect(welsh_translation_input.support_email_cy).to eq("welsh-support@example.gov.uk")
+      end
+    end
+
+    context "when the spreadsheet data includes blank values" do
+      let(:spreadsheet_data) do
+        {
+          "Form name" => "Welsh A form from spreadsheet",
+          "Contact details for support - email address" => "",
+        }
+      end
+
+      it "uses the Welsh already set on the form for fields with blank values in the spreadsheet data" do
+        welsh_translation_input = described_class.new(form:)
+        welsh_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_translation_input.name_cy).to eq("Welsh A form from spreadsheet")
+        expect(welsh_translation_input.support_email_cy).to eq("welsh-support@example.gov.uk")
+      end
+    end
+  end
+
   describe "#all_fields_empty?" do
     context "when the welsh translation fields are not empty" do
       let(:form) { build_form }
diff --git a/spec/input_objects/forms/welsh_translation_input_spec.rb b/spec/input_objects/forms/welsh_translation_input_spec.rb
index cebb22d993..427a7816d9 100644
--- a/spec/input_objects/forms/welsh_translation_input_spec.rb
+++ b/spec/input_objects/forms/welsh_translation_input_spec.rb
@@ -12,7 +12,7 @@
            guidance_markdown: "This part of the form concerns licencing.",
            position: 1
   end
-  let(:another_page) { create :page }
+  let(:another_page) { create :page, position: 2 }
 
   let(:mark_complete) { "true" }
 
@@ -89,6 +89,13 @@ def build_empty_welsh_form
       expect(welsh_translation_input.errors.full_messages_for(:mark_complete)).to include "Mark complete #{I18n.t('activemodel.errors.models.forms/welsh_translation_input.attributes.mark_complete.blank')}"
     end
 
+    it "is valid if mark complete is blank and the scope is :upload" do
+      form = OpenStruct.new(welsh_completed: false, name: "Apply for a juggling licence")
+      welsh_translation_input = described_class.new(mark_complete: nil, form:)
+
+      expect(welsh_translation_input.valid?(:upload)).to be true
+    end
+
     context "when the form is marked complete" do
       let(:mark_complete) { "true" }
 
@@ -692,6 +699,93 @@ def build_empty_welsh_form
     end
   end
 
+  describe "#assign_from_spreadsheet" do
+    before do
+      form.pages = [page, another_page]
+    end
+
+    context "when the spreadsheet data contains Welsh translations for all fields" do
+      let(:spreadsheet_data) do
+        {
+          "Form name" => "Welsh A form from spreadsheet",
+          "Question 1 - question text" => "Welsh question text from spreadsheet",
+          "Question 1 - option 1" => "Welsh Option 1 from spreadsheet",
+          "Question 1 - option 2" => "Welsh Option 2 from spreadsheet",
+          "Question 1 - question or label if 'None of the above' is selected" => "Welsh None of the above question? from spreadsheet",
+          "Question 1 - exit page heading" => "Welsh exit page heading from spreadsheet",
+          "Question 1 - exit page content" => "Welsh exit page content from spreadsheet",
+          "Question 2 - exit page 1 heading" => "Welsh question 2 exit page heading from spreadsheet",
+          "Question 2 - exit page 1 content" => "Welsh question 2 exit page content from spreadsheet",
+          "Question 2 - page heading" => "Welsh question 2 page heading from spreadsheet",
+          "Question 2 - guidance text" => "Welsh question 2 guidance markdown from spreadsheet",
+          "Question 2 - question text" => "Welsh question 2 text from spreadsheet",
+          "Declaration" => "Welsh declaration text from spreadsheet",
+          "Information about what happens next" => "Welsh what happens next from spreadsheet",
+          "GOV.UK Pay payment link" => "https://www.gov.uk/payment_cy_spreadsheet",
+          "Link to privacy information for this form" => "https://www.gov.uk/privacy_cy_spreadsheet",
+          "Contact details for support - email address" => "support-spreadsheet@example.gov.wales",
+          "Contact details for support - phone number and opening times" => "Welsh support phone from spreadsheet",
+          "Contact details for support - online contact link" => "https://www.gov.uk/support_cy_spreadsheet",
+          "Contact details for support - online contact link text" => "Welsh Support URL text from spreadsheet",
+        }
+      end
+
+      it "sets the welsh attributes from the spreadsheet data" do
+        welsh_translation_input = described_class.new(form:)
+        welsh_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_translation_input.name_cy).to eq("Welsh A form from spreadsheet")
+        expect(welsh_translation_input.declaration_markdown_cy).to eq("Welsh declaration text from spreadsheet")
+        expect(welsh_translation_input.what_happens_next_markdown_cy).to eq("Welsh what happens next from spreadsheet")
+        expect(welsh_translation_input.payment_url_cy).to eq("https://www.gov.uk/payment_cy_spreadsheet")
+        expect(welsh_translation_input.privacy_policy_url_cy).to eq("https://www.gov.uk/privacy_cy_spreadsheet")
+        expect(welsh_translation_input.support_email_cy).to eq("support-spreadsheet@example.gov.wales")
+        expect(welsh_translation_input.support_phone_cy).to eq("Welsh support phone from spreadsheet")
+        expect(welsh_translation_input.support_url_cy).to eq("https://www.gov.uk/support_cy_spreadsheet")
+        expect(welsh_translation_input.support_url_text_cy).to eq("Welsh Support URL text from spreadsheet")
+
+        page_translation = welsh_translation_input.page_translations.find { |pt| pt.id == page.id }
+        expect(page_translation.question_text_cy).to eq("Welsh question text from spreadsheet")
+
+        another_page_translation = welsh_translation_input.page_translations.find { |pt| pt.id == another_page.id }
+        expect(another_page_translation.page_heading_cy).to eq("Welsh question 2 page heading from spreadsheet")
+      end
+    end
+
+    context "when the spreadsheet data does not include keys for all fields" do
+      let(:spreadsheet_data) do
+        {
+          "Form name" => "Welsh A form from spreadsheet",
+        }
+      end
+
+      it "uses the Welsh already set on the form for fields not present in the spreadsheet data" do
+        welsh_translation_input = described_class.new(form:)
+        welsh_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_translation_input.name_cy).to eq("Welsh A form from spreadsheet")
+        expect(welsh_translation_input.support_email_cy).to eq("welsh-support@example.gov.uk")
+      end
+    end
+
+    context "when the spreadsheet data includes blank values" do
+      let(:spreadsheet_data) do
+        {
+          "Form name" => "Welsh A form from spreadsheet",
+          "Contact details for support - email address" => "",
+        }
+      end
+
+      it "uses the Welsh already set on the form for fields with blank values in the spreadsheet data" do
+        welsh_translation_input = described_class.new(form:)
+        welsh_translation_input.assign_from_spreadsheet(spreadsheet_data)
+
+        expect(welsh_translation_input.name_cy).to eq("Welsh A form from spreadsheet")
+        expect(welsh_translation_input.support_email_cy).to eq("welsh-support@example.gov.uk")
+      end
+    end
+  end
+
   describe "#all_fields_empty?" do
     context "when the welsh translation fields are not empty" do
       let(:form) { build_form }
diff --git a/spec/input_objects/forms/welsh_translation_upload_input_spec.rb b/spec/input_objects/forms/welsh_translation_upload_input_spec.rb
new file mode 100644
index 0000000000..031a487914
--- /dev/null
+++ b/spec/input_objects/forms/welsh_translation_upload_input_spec.rb
@@ -0,0 +1,100 @@
+require "rails_helper"
+
+RSpec.describe Forms::WelshTranslationUploadInput do
+  let(:form) { create(:form) }
+
+  describe "validations" do
+    it "is invalid without a file" do
+      input = described_class.new(form: form, file: nil)
+      expect(input).to be_invalid
+      expect(input.errors.full_messages_for(:file)).to include("File Select a file")
+    end
+
+    it "is invalid with an unsupported file type" do
+      file = fixture_file_upload("invalid.txt", "text/plain")
+      input = described_class.new(form: form, file: file)
+      expect(input).to be_invalid
+      expect(input.errors.full_messages_for(:file)).to include("File The selected file must be a CSV")
+    end
+
+    it "is invalid with a file that exceeds the maximum size" do
+      file = fixture_file_upload("valid.csv", "text/csv")
+      allow(file).to receive(:size).and_return((Forms::WelshTranslationUploadInput::MAX_SIZE_IN_MB + 1).megabytes)
+
+      input = described_class.new(form: form, file: file)
+      expect(input).to be_invalid
+      expect(input.errors.full_messages_for(:file)).to include("File The selected file must be smaller than 10MB")
+    end
+
+    it "is valid with a supported file type" do
+      Tempfile.create do |file|
+        file.write("header1,header2\nvalue1,value2")
+        file.rewind
+
+        input = described_class.new(form: form, file: Rack::Test::UploadedFile.new(file, "text/csv"))
+        expect(input).to be_valid
+      end
+    end
+  end
+
+  describe "#read_file" do
+    let(:mock_welsh_csv_import_service) { instance_double(WelshCsvImportService) }
+    let(:file) do
+      Tempfile.create do |file|
+        file.write("header1,header2\nvalue1,value2")
+        file.rewind
+        Rack::Test::UploadedFile.new(file, "text/csv")
+      end
+    end
+
+    before do
+      allow(WelshCsvImportService).to receive(:new).and_return(mock_welsh_csv_import_service)
+    end
+
+    after do
+      file.unlink
+    end
+
+    it "returns false when the input is invalid" do
+      input = described_class.new(form: form, file: nil)
+      expect(input.read_file).to be false
+    end
+
+    context "when the CSV is valid" do
+      let(:translations) { { "key1" => "translation1", "key2" => "translation2" } }
+
+      before do
+        allow(mock_welsh_csv_import_service).to receive(:read).and_return(translations)
+      end
+
+      it "returns the translations from the CSV" do
+        input = described_class.new(form: form, file: file)
+        expect(input.read_file).to eq(translations)
+      end
+    end
+
+    context "when the CSV is malformed" do
+      before do
+        allow(mock_welsh_csv_import_service).to receive(:read).and_raise(CSV::MalformedCSVError.new("error", 1))
+      end
+
+      it "returns false and adds an error" do
+        input = described_class.new(form: form, file: file)
+        expect(input.read_file).to be false
+        expect(input.errors.full_messages_for(:file)).to include("File The CSV has invalid formatting - try downloading a new version, then uploading it again")
+      end
+    end
+
+    context "when the CSV has invalid headers" do
+      before do
+        allow(mock_welsh_csv_import_service).to receive(:read).and_raise(WelshCsvImportService::InvalidHeadersError)
+      end
+
+      it "returns false and adds an error" do
+        input = described_class.new(form: form, file: file)
+        expect(input.read_file).to be false
+        expect(input.errors.full_messages_for(:file)).to include("File We couldn’t upload the CSV because the column headings are wrong - check them and try uploading again")
+      end
+    end
+  end
+end
diff --git a/spec/requests/forms/welsh_translation_controller_spec.rb b/spec/requests/forms/welsh_translation_controller_spec.rb
index 8df87a54e5..d796c88ec3 100644
--- a/spec/requests/forms/welsh_translation_controller_spec.rb
+++ b/spec/requests/forms/welsh_translation_controller_spec.rb
@@ -46,8 +46,8 @@
           expect {
             post(welsh_translation_create_path(id), params:)
           }.to change { form.reload.welsh_completed }.to(true)
-          .and change { form.pages.first.reload.question_text_cy }.to("Ydych chi'n adnewyddu trwydded?")
-          .and change { condition.reload.exit_page_heading_cy }.to("Nid ydych yn gymwys")
+                                                     .and change { form.pages.first.reload.question_text_cy }.to("Ydych chi'n adnewyddu trwydded?")
+                                                                                                             .and change { condition.reload.exit_page_heading_cy }.to("Nid ydych yn gymwys")
         end
 
         it "redirects to the form task list and displays a success banner including text about being marked complete" do
@@ -81,8 +81,8 @@
           expect {
             post(welsh_translation_create_path(id), params:)
           }.to not_change { form.reload.welsh_completed }
-          .and not_change { form.pages.first.reload.question_text_cy }
-          .and(not_change { condition.reload.exit_page_markdown_cy })
+                 .and not_change { form.pages.first.reload.question_text_cy }
+                        .and(not_change { condition.reload.exit_page_markdown_cy })
         end
 
         it "returns a 422, re-renders the page with an error, and does not display a success banner" do
@@ -149,8 +149,8 @@
           expect {
             post(welsh_translation_create_path(id), params:)
           }.to not_change { form.reload.welsh_completed }
-          .and not_change { form.pages.first.reload.question_text_cy }
-          .and(not_change { condition.reload.exit_page_markdown_cy })
+                 .and not_change { form.pages.first.reload.question_text_cy }
+                        .and(not_change { condition.reload.exit_page_markdown_cy })
         end
 
         it "returns 403" do
@@ -172,8 +172,8 @@
           expect {
             post(welsh_translation_create_path(id), params:)
           }.to change { form.reload.welsh_completed }.to(true)
-          .and change { form.pages.first.reload.question_text_cy }.to("Ydych chi'n adnewyddu trwydded?")
-          .and change { exit_page.reload.heading_cy }.to("Nid ydych yn gymwys")
+                                                     .and change { form.pages.first.reload.question_text_cy }.to("Ydych chi'n adnewyddu trwydded?")
+                                                                                                             .and change { exit_page.reload.heading_cy }.to("Nid ydych yn gymwys")
         end
 
         it "redirects to the form task list and displays a success banner including text about being marked complete" do
@@ -207,8 +207,8 @@
           expect {
             post(welsh_translation_create_path(id), params:)
           }.to not_change { form.reload.welsh_completed }
-          .and not_change { form.pages.first.reload.question_text_cy }
-          .and(not_change { exit_page.reload.markdown_cy })
+                 .and not_change { form.pages.first.reload.question_text_cy }
+                        .and(not_change { exit_page.reload.markdown_cy })
         end
 
         it "returns a 422, re-renders the page with an error, and does not display a success banner" do
@@ -275,8 +275,8 @@
           expect {
             post(welsh_translation_create_path(id), params:)
           }.to not_change { form.reload.welsh_completed }
-          .and not_change { form.pages.first.reload.question_text_cy }
-          .and(not_change { exit_page.reload.markdown_cy })
+                 .and not_change { form.pages.first.reload.question_text_cy }
+                        .and(not_change { exit_page.reload.markdown_cy })
         end
 
         it "returns 403" do
@@ -423,8 +423,165 @@
     it "returns a CSV with a header row and and content" do
       csv = CSV.parse(response.body)
 
-      expect(csv.first).to eq(["", "English content", "Welsh content"])
+      expect(csv.first).to eq(["Content ID", "English content", "Welsh content"])
       expect(csv.second).to eq(["Form name", "A form with Welsh", "Welsh A form with Welsh"])
     end
   end
+
+  describe "#show_upload" do
+    before do
+      get welsh_translation_show_upload_path(id)
+    end
+
+    it "renders the template" do
+      expect(response).to have_http_status(:ok)
+      expect(response).to render_template(:show_upload)
+    end
+
+    context "when the user is not authorized" do
+      let(:current_user) { build :user }
+
+      it "returns 403" do
+        expect(response).to have_http_status(:forbidden)
+      end
+    end
+  end
+
+  describe "#upload" do
+    let(:form) { create(:form, :ready_for_live, welsh_completed: false) }
+    let(:csv_data) { "foo,bar" }
+    let(:file) do
+      file = Tempfile.new(["translations", ".csv"])
+      file.write(csv_data)
+      file.rewind
+      Rack::Test::UploadedFile.new(file.path, "text/csv", original_filename: "translations.csv")
+    end
+
+    after do
+      file.unlink
+    end
+
+    context "when a valid CSV is uploaded" do
+      context "when the multiple branches feature is disabled", feature_multiple_branches: false do
+        let(:condition) do
+          create(:condition, routing_page: form.pages.first, answer_value: "No",
+                             exit_page_heading: "You are ineligible",
+                             exit_page_markdown: "Sorry, you are ineligible for this service.")
+        end
+
+        let(:csv_data) do
+          CSV.generate do |csv|
+            csv << ["Content ID", "English content", "Welsh content"]
+            csv << ["Form name", form.name, "Fy Ffurflen"]
+            csv << ["Question 1 - exit page heading", "You are ineligible", "Welsh exit page heading"]
+            csv << ["Question 1 - exit page content", "Sorry, you are ineligible for this service.", "Welsh exit page content"]
+          end
+        end
+
+        before do
+          condition
+          post welsh_translation_upload_path(id), params: { forms_welsh_translation_upload_input: { file: } }
+        end
+
+        it "renders the new template" do
+          expect(response).to have_http_status(:ok)
+          expect(response).to render_template(:new)
+          expect(response.body).to include(I18n.t("banner.success.form.welsh_translation_uploaded"))
+        end
+
+        it "pre-populates fields from CSV" do
+          expect(response.body).not_to include(I18n.t("error_summary.heading"))
+          expect(response.body).to include("Fy Ffurflen")
+          expect(response.body).to include("Welsh exit page heading")
+          expect(response.body).to include("Welsh exit page content")
+        end
+
+        context "when there are invalid translations" do
+          let(:csv_data) do
+            CSV.generate do |csv|
+              csv << ["Content ID", "English content", "Welsh content"]
+              csv << ["Link to privacy information for this form", form.privacy_policy_url, "this is not a URL"]
+            end
+          end
+
+          it "pre-validates the fields and shows errors for invalid data" do
+            expect(response.body).to include(I18n.t("error_summary.heading"))
+            expect(response.body).to include(I18n.t("activemodel.errors.models.forms/welsh_translation_input.attributes.privacy_policy_url_cy.url"))
+            expect(response.body).to include("this is not a URL")
+          end
+        end
+      end
+
+      context "when the multiple branches feature is enabled", :feature_multiple_branches do
+        let(:exit_page) { create :exit_page, question_page: form.pages.first }
+
+        let(:csv_data) do
+          CSV.generate do |csv|
+            csv << ["Content ID", "English content", "Welsh content"]
+            csv << ["Form name", form.name, "Fy Ffurflen"]
+            csv << ["Question 1 - exit page 1 heading", "You are ineligible", "Welsh exit page heading"]
+            csv << ["Question 1 - exit page 1 content", "Sorry, you are ineligible for this service.", "Welsh exit page content"]
+          end
+        end
+
+        before do
+          exit_page
+          post welsh_translation_upload_path(id), params: { forms_welsh_translation_upload_input: { file: } }
+        end
+
+        it "renders the new template" do
+          expect(response).to have_http_status(:ok)
+          expect(response).to render_template(:new)
+          expect(response.body).to include(I18n.t("banner.success.form.welsh_translation_uploaded"))
+        end
+
+        it "pre-populates fields from CSV" do
+          expect(response.body).not_to include(I18n.t("error_summary.heading"))
+          expect(response.body).to include("Fy Ffurflen")
+          expect(response.body).to include("Welsh exit page heading")
+          expect(response.body).to include("Welsh exit page content")
+        end
+
+        context "when there are invalid translations" do
+          let(:csv_data) do
+            CSV.generate do |csv|
+              csv << ["Content ID", "English content", "Welsh content"]
+              csv << ["Link to privacy information for this form", form.privacy_policy_url, "this is not a URL"]
+            end
+          end
+
+          it "pre-validates the fields and shows errors for invalid data" do
+            expect(response.body).to include(I18n.t("error_summary.heading"))
+            expect(response.body).to include(I18n.t("activemodel.errors.models.forms/welsh_translation_input.attributes.privacy_policy_url_cy.url"))
+            expect(response.body).to include("this is not a URL")
+          end
+        end
+      end
+
+      context "when no file is provided" do
+        before do
+          post welsh_translation_upload_path(id), params: { forms_welsh_translation_upload_input: { file: nil } }
+        end
+
+        it "renders the upload file page with an error" do
+          expect(response).to have_http_status(:unprocessable_content)
+          expect(response).to render_template(:show_upload)
+          expect(response.body).to include(I18n.t("activemodel.errors.models.forms/welsh_translation_upload_input.attributes.file.blank"))
+          expect(flash).to be_empty
+        end
+      end
+    end
+
+    context "when the user is not authorized" do
+      let(:current_user) { build :user }
+
+      before do
+        post welsh_translation_upload_path(id), params: { file: }
+      end
+
+      it "returns 403" do
+        expect(response).to have_http_status(:forbidden)
+      end
+    end
+  end
 end
diff --git a/spec/services/welsh_csv_import_service_spec.rb b/spec/services/welsh_csv_import_service_spec.rb
new file mode 100644
index 0000000000..7594a8c2ad
--- /dev/null
+++ b/spec/services/welsh_csv_import_service_spec.rb
@@ -0,0 +1,128 @@
+require "rails_helper"
+
+RSpec.describe WelshCsvImportService do
+  subject(:service) { described_class.new(file, form) }
+
+  let(:form) do
+    create :form,
+           :with_pages,
+           name: "A form",
+           what_happens_next_markdown: "English what happens next",
+           privacy_policy_url: "https://www.gov.uk/privacy",
+           payment_url: "https://www.gov.uk/payment",
+           support_email: "support@example.gov.uk",
+           support_phone: "English support phone",
+           support_url: "https://www.gov.uk/support",
+           support_url_text: "Support URL text",
+           declaration_text: "Declaration text",
+           pages: [page, another_page]
+  end
+  let(:page) do
+    create :page,
+           :selection_with_none_of_the_above_question,
+           question_text: "None of the above question?",
+           none_of_the_above_question_text: "None of the above question?",
+           routing_conditions: [condition]
+  end
+  let(:condition) { create :condition, :with_exit_page, exit_page_heading: "Exit page heading" }
+  let(:another_page) { create :page, question_text: "What?", page_heading: "Page heading", guidance_markdown: "This is the guidance." }
+
+  let(:file) { Tempfile.new }
+
+  after do
+    file.unlink
+  end
+
+  context "when the CSV is valid" do
+    context "when the rows in the CSV match the current form" do
+      let(:bom) { "" }
+
+      before do
+        rows = [
+          ["#{bom}Content ID", "English content", "Welsh content"],
+          ["Form name", "A form", "Welsh A form ôÂŵéï"],
+          ["Question 1 - question text", "None of the above question?", "Welsh None of the above question?"],
+          ["Question 1 - option 1", "Option 1", "Welsh Option 1"],
+          ["Question 1 - option 2", "Option 2", "Welsh Option 2"],
+          ["Question 1 - question or label if 'None of the above' is selected", "None of the above question?", "Welsh None of the above question?"],
+          ["Question 1 - exit page heading", "Exit page heading", "Welsh exit page heading"],
+          ["Question 1 - exit page content", "Exit page markdown", "Welsh exit page markdown"],
+          ["Question 2 - page heading", "Page heading", "Welsh Page heading"],
+          ["Question 2 - guidance text", "This is the guidance.", "Welsh This is the guidance."],
+          ["Question 2 - question text", "What?", "Welsh What?"],
+          ["Declaration", "Declaration text", "Welsh declaration text"],
+          ["Information about what happens next", "English what happens next", "Welsh what happens next"],
+          ["GOV.UK Pay payment link", "https://www.gov.uk/payment", "https://www.gov.uk/payment_cy"],
+          ["Link to privacy information for this form", "https://www.gov.uk/privacy", "https://www.gov.uk/privacy_cy"],
+          ["Contact details for support - email address", "support@example.gov.uk", "support@example.gov.uk"],
+          ["Contact details for support - phone number and opening times", "English support phone", "Welsh support phone"],
+          ["Contact details for support - online contact link", "https://www.gov.uk/support", "https://www.gov.uk/support_cy"],
+          ["Contact details for support - online contact link text", "Support URL text", "Welsh Support URL text"],
+        ]
+        file.write(rows.map(&:to_csv).join)
+        file.rewind
+      end
+
+      it "returns the translations data" do
+        expect(service.read).to eq({
+          "Form name" => "Welsh A form ôÂŵéï",
+          "Question 1 - question text" => "Welsh None of the above question?",
+          "Question 1 - option 1" => "Welsh Option 1",
+          "Question 1 - option 2" => "Welsh Option 2",
+          "Question 1 - question or label if 'None of the above' is selected" => "Welsh None of the above question?",
+          "Question 1 - exit page heading" => "Welsh exit page heading",
+          "Question 1 - exit page content" => "Welsh exit page markdown",
+          "Question 2 - page heading" => "Welsh Page heading",
+          "Question 2 - guidance text" => "Welsh This is the guidance.",
+          "Question 2 - question text" => "Welsh What?",
+          "Declaration" => "Welsh declaration text",
+          "Information about what happens next" => "Welsh what happens next",
+          "GOV.UK Pay payment link" => "https://www.gov.uk/payment_cy",
+          "Link to privacy information for this form" => "https://www.gov.uk/privacy_cy",
+          "Contact details for support - email address" => "support@example.gov.uk",
+          "Contact details for support - phone number and opening times" => "Welsh support phone",
+          "Contact details for support - online contact link" => "https://www.gov.uk/support_cy",
+          "Contact details for support - online contact link text" => "Welsh Support URL text",
+        })
+      end
+
+      context "when the CSV has a BOM" do
+        let(:bom) { "\uFEFF" }
+
+        it "returns the translations data" do
+          expect(service.read).to include("Form name" => "Welsh A form ôÂŵéï")
+        end
+      end
+    end
+  end
+
+  context "when the CSV has invalid headers" do
+    before do
+      rows = [
+        ["Content ID", "Unexpected", "Welsh content"],
+        ["Form name", "A form", "Welsh A form"],
+      ]
+      file.write(rows.map(&:to_csv).join)
+      file.rewind
+    end
+
+    it "raises an InvalidHeadersError" do
+      expect { service.read }.to raise_error(WelshCsvImportService::InvalidHeadersError)
+    end
+  end
+
+  context "when the headers are in the wrong order" do
+    before do
+      rows = [
+        ["Welsh content", "English content", "Content ID"],
+        ["Form name", "Welsh A form", "A form"],
+      ]
+      file.write(rows.map(&:to_csv).join)
+      file.rewind
+    end
+
+    it "raises an InvalidHeadersError" do
+      expect { service.read }.to raise_error(WelshCsvImportService::InvalidHeadersError)
+    end
+  end
+end
diff --git a/spec/services/welsh_csv_service_spec.rb b/spec/services/welsh_csv_service_spec.rb
index 613807c067..c3ae445597 100644
--- a/spec/services/welsh_csv_service_spec.rb
+++ b/spec/services/welsh_csv_service_spec.rb
@@ -4,12 +4,8 @@
   describe "#as_csv" do
     let(:form) { build :form }
 
-    it "contains the header row" do
-      expect(csv_rows(form)[0]).to contain_exactly(
-        "",
-        "English content",
-        "Welsh content",
-      )
+    it "contains the header row, including the UTF-8 BOM" do
+      expect(csv_rows(form)[0]).to eq(["\uFEFFContent ID", "English content", "Welsh content"])
     end
 
     it "contains the form name" do
@@ -49,7 +45,7 @@
 
       it "contains the payment URL" do
         expect(csv_rows(form)).to include([
-          "GOV⁠.⁠UK Pay payment link",
+          "GOV.UK Pay payment link",
           "https://www.gov.uk/payment",
           "https://www.gov.uk/payment_cy",
         ])
@@ -201,7 +197,7 @@
 
       it "contains the none of the above question" do
         expect(csv_rows(form)).to include([
-          "Question 1 - question or label if ‘None of the above’ is selected",
+          "Question 1 - question or label if 'None of the above' is selected",
           "None of the above question?",
           "Welsh None of the above question?",
         ])
@@ -347,12 +343,12 @@
       it "returns a CSV with a header row and the expected rows" do
         csv = csv_rows(form)
 
-        expected_csv = [["", "English content", "Welsh content"],
+        expected_csv = [["\uFEFFContent ID", "English content", "Welsh content"],
                         ["Form name", "A form", "Welsh A form"],
                         ["Question 1 - question text", "None of the above question?", "Welsh None of the above question?"],
                         ["Question 1 - option 1", "Option 1", "Option 1"],
                         ["Question 1 - option 2", "Option 2", "Option 2"],
-                        ["Question 1 - question or label if ‘None of the above’ is selected", "None of the above question?", "Welsh None of the above question?"],
+                        ["Question 1 - question or label if 'None of the above' is selected", "None of the above question?", "Welsh None of the above question?"],
                         ["Question 1 - exit page heading", "Exit page heading", "Welsh exit page heading"],
                         ["Question 1 - exit page content", "Exit page markdown", "Welsh exit page markdown"],
                         ["Question 2 - page heading", "Page heading", "Welsh Page heading"],
@@ -360,7 +356,7 @@
                         ["Question 2 - question text", "What?", "Welsh What?"],
                         ["Declaration", "Declaration text", ""],
                         ["Information about what happens next", "English what happens next", "Welsh what happens next"],
-                        ["GOV⁠.⁠UK Pay payment link", "https://www.gov.uk/payment", "https://www.gov.uk/payment_cy"],
+                        ["GOV.UK Pay payment link", "https://www.gov.uk/payment", "https://www.gov.uk/payment_cy"],
                         ["Link to privacy information for this form", "https://www.gov.uk/privacy", ""],
                         ["Contact details for support - email address", "support@example.gov.uk", "support@example.gov.uk"],
                         ["Contact details for support - phone number and opening times", "English support phone", "Welsh support phone"],
diff --git a/spec/views/forms/welsh_translation/show_upload.html.erb_spec.rb b/spec/views/forms/welsh_translation/show_upload.html.erb_spec.rb
new file mode 100644
index 0000000000..8e4b795eab
--- /dev/null
+++ b/spec/views/forms/welsh_translation/show_upload.html.erb_spec.rb
@@ -0,0 +1,44 @@
+require "rails_helper"
+
+describe "forms/welsh_translation/show_upload.html.erb" do
+  let(:form) { create :form }
+  let(:welsh_translation_upload_input) { Forms::WelshTranslationUploadInput.new(form: form) }
+
+  before do
+    render template: "forms/welsh_translation/show_upload", locals: {
+      current_form: form,
+      welsh_translation_upload_input: welsh_translation_upload_input,
+    }
+  end
+
+  it "contains a top-level heading" do
+    expect(rendered).to have_css("h1", text: I18n.t("page_titles.welsh_translation_upload"))
+  end
+
+  it "has a back link to the welsh translation page" do
+    expect(view.content_for(:back_link)).to have_link("Back", href: welsh_translation_path(form))
+  end
+
+  it "contains a form for uploading a Welsh translation CSV file" do
+    expect(rendered).to have_css("form[action='#{welsh_translation_upload_path(form)}'][method='post'][enctype='multipart/form-data']")
+  end
+
+  it "contains a file input field for uploading the CSV file" do
+    expect(rendered).to have_css("input[type='file'][name='forms_welsh_translation_upload_input[file]'][accept='text/csv']")
+  end
+
+  context "when the form has errors" do
+    before do
+      welsh_translation_upload_input.errors.add(:file, "an error occurred")
+      render template: "forms/welsh_translation/show_upload", locals: {
+        current_form: form,
+        welsh_translation_upload_input: welsh_translation_upload_input,
+      }
+    end
+
+    it "displays the error message" do
+      expect(rendered).to have_css(".govuk-error-summary")
+      expect(rendered).to have_css(".govuk-error-message", text: "an error occurred")
+    end
+  end
+end