wpml-rest-api.php

<?php

/*
Plugin Name: WPML REST API
Version: 1.0
Description: Adds links to posts in other languages into the results of a WP REST API query for sites running the WPML plugin.
Author: Shawn Hooper
Author URI: https://profiles.wordpress.org/shooper
*/

add_action( 'rest_api_init', 'wpmlrestapi_init', 1000);

function wpmlrestapi_init() {

	// Check if WPML is installed
	include_once( ABSPATH . 'wp-admin/includes/plugin.php' );

	if (!is_plugin_active('sitepress-multilingual-cms/sitepress.php')) {
		return;
	}
	
	$available_langs = wpml_get_active_languages_filter('', array('skip_missing' => false, ) );
		
	if ( ! empty( $available_langs ) && ! isset( $GLOBALS['icl_language_switched'] ) || ! $GLOBALS['icl_language_switched'] ) {
		if ( isset( $_REQUEST['wpml_lang'] ) ) {
			$lang = $_REQUEST['wpml_lang'];
		} else if ( isset( $_REQUEST['lang'] ) ) {
			$lang = $_REQUEST['lang'];
		}
		
		if ( isset( $lang ) && in_array( $lang, array_keys( $available_langs ) ) ) {
			do_action( 'wpml_switch_language', $lang );
		}
	}

	// Add WPML fields to all post types
	// Thanks to Roy Sivan for this trick.
	// http://www.roysivan.com/wp-api-v2-adding-fields-to-all-post-types/#.VsH0e5MrLcM

	$post_types = get_post_types( array( 'public' => true, 'exclude_from_search' => false ), 'names' );
	foreach( $post_types as $post_type ) {
		wpmlrestapi_register_api_field($post_type);
	}
}

function wpmlrestapi_register_api_field($post_type) {
	register_rest_field( $post_type,
		'wpml_current_locale',
		array(
			'get_callback'    => 'wpmlrestapi_slug_get_current_locale',
			'update_callback' => null,
			'schema'          => null,
		)
	);

	register_rest_field( $post_type,
		'wpml_translations',
		array(
			'get_callback'    => 'wpmlrestapi_slug_get_translations',
			'update_callback' => null,
			'schema'          => null,
		)
	);
}

/**
* Retrieve available translations
*
* @param array $object Details of current post.
* @param string $field_name Name of field.
* @param WP_REST_Request $request Current request
*
* @return mixed
*/
function wpmlrestapi_slug_get_translations( $object, $field_name, $request ) {
	global $sitepress;
	$languages = apply_filters('wpml_active_languages', null);
	$translations = [];

	foreach ($languages as $language) {
		$post_id = wpml_object_id_filter($object['id'], 'post', false, $language['language_code']);
		if ($post_id === null || $post_id == $object['id']) continue;
		$thisPost = get_post($post_id);

		$href= apply_filters( 'WPML_filter_link', $language[ 'url' ], $language );
		if (strpos($href, '?') !== false) {
			$href = str_replace('?', '/' . $thisPost->post_name . '/?', $href);
		} else {

			if (substr($href, -1) !== '/') {
				$href .= '/';
			}

			$href .= $thisPost->post_name . '/';
		}

		$translations[] = array('locale' => $language['default_locale'], 'id' => $thisPost->ID, 'post_title' => $thisPost->post_title, 'href' => $href);
	}

	return $translations;
}

/**
 * Retrieve the current locale
 *
 * @param array $object Details of current post.
 * @param string $field_name Name of field.
 * @param WP_REST_Request $request Current request
 *
 * @return mixed
 */
function wpmlrestapi_slug_get_current_locale( $object, $field_name, $request ) {
	$langInfo = wpml_get_language_information($object);
	return $langInfo['locale'];
}