CKEditor 5 should explicitly set negotiated content language, not just UI language

Problem/Motivation

While testing a contributed module's CKEditor 5 support, @anairamzap remarked:

The only thing missing to behave exactly as is CK4 is not related with this plugin/module, but with the contentsLangDirection https://docs-old.ckeditor.com/ckeditor_api/symbols/CKEDITOR.config.html#... that we were using to set rtl as the default direction. Just commenting this here in case it helps someone else for the upgrade: https://ckeditor.com/docs/ckeditor5/latest/installation/getting-started/...
UPDATE: to set the default language/direction in CK5 you can use the following hook:

/**
 * Implements hook_editor_js_settings_alter.
 */
function module_name_editor_js_settings_alter(array &$settings) {
  foreach ($settings['editor']['formats'] as $name => $value) {
    $settings['editor']['formats'][$name]['editorSettings']['language'] = [
      'ui' => 'en',
      'content' => 'ar'
    ];
  }
}

That uses the language config for CK5 API and sets a different lang for the UI and the editor content for all editor formats.

The odd thing is that Drupal core never configured CKEditor 4's contentsLangDirection. So AFAICT this can't be a regression. Unless there's some undocumented behavior in CKEditor 4 that automatically applied this; I suspect that it's really just the CKEditor 5 BiDi plugin does not set the content language direction correctly? (That was a contributed module/plugin in CKEditor 4 too: https://www.drupal.org/project/ckeditor_bidi → — so not a core bug then.)

But, Drupal core natively supports distinguishing UI language (\Drupal\Core\Language\LanguageInterface::TYPE_INTERFACE) and content language (\Drupal\Core\Language\LanguageInterface::TYPE_CONTENT). This cleanly maps onto CKEditor 5's capabilities.

Today, we already make sure the CKEditor 5 UI language matches Drupal's UI language, in \Drupal\ckeditor5\Plugin\Editor\CKEditor5::getJSSettings():

    if ($this->moduleHandler->moduleExists('locale')) {
      $language_interface = $this->languageManager->getCurrentLanguage();
      $settings['language']['ui'] = _ckeditor5_get_langcode_mapping($language_interface->getId());
    }

Steps to reproduce

TBD

Proposed resolution

Add something like

    if ($this->moduleHandler->moduleExists('content_translation')) {
      $content_language = $this->languageManager->getCurrentLanguage(LanguageInterface::TYPE_CONTENT);
      $settings['language']['content'] = _ckeditor5_get_langcode_mapping($content_language->getId());
    }

🤔🤓 But what about multiple content languages on a single page? Does CKEditor 5 support specifying a different content language per instance? Does the editor.module API allow for that? That seems like it'd need a follow-up.

Remaining tasks

1. Failing test with negotiated content language not matching negotiated UI language
2. Fix
3. Discuss multiple distinct content languages in use in a single response, regardless of negotiation. Consider opening up follow-up?

User interface changes

TBD

API changes

None.

Data model changes

None.

Release notes snippet

TBD