Issue #3514280: Add Translations processor to import entity translations
To possibly preserve the entity's original language, the processor imports the original language in the prepare_importable_entity_data stage, i.e. before the actual requested entity is imported. All other translations are imported after the requested entity is finished, in the post_entity_save stage.
Since the JSON:API does not yet provide a language negotiation mechanism, this implementation relies on the language-content-entity mechanism, using the language_content_entity URL query parameter to query different languages.
For more information on the JSON:API language negotiation issue see:
https://www.drupal.org/project/drupal/issues/2794431Closes #3514280