[codex] add default language sync for control panel

[sneridagh]

What changed

• Added a control-panel-only field deserializer for ILanguageSchema.default_language.
• Syncs the site language when @controlpanels/language changes default_language on non-multilingual sites.
• Leaves multilingual sites untouched.
• Split the language-specific tests into test_services_controlpanel_language.py and added a towncrier fragment.

Why

• Control panel deserialization uses a fake Dexterity context, so the language control panel needed request-scoped behavior to update the actual portal language.

Validation

• `.venv/bin/zope-testrunner --all --test-path=src -s plone.restapi -m 'test_services_controlpanel_language|test_services_controlpanels'

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📚 Documentation preview 📚: https://plonerestapi--2021.org.readthedocs.build/

[mister-roboto]

@sneridagh thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[sneridagh]

@jenkins-plone-org please run jobs

[sneridagh]

@davisagli No clue why all these failures...

[davisagli]

@sneridagh The desired feature here isn't specific to the REST API. It should happen when the default language is changed via any UI. So instead of implementing it in a deserializer, I think we should add this to what is taken care of by this event handler in plone.app.multilingual: https://github.com/plone/plone.app.multilingual/blob/master/src/plone/app/multilingual/subscriber.py#L184