Translations

JSON:API supports very simple multilingual functionality. It does not support advanced use cases.

It currently relies on Drupal's default language negotiation mechanisms to negotiate the appropriate language of an entity and fall back when that translation is not available. In the long term, we intend to move away from this mechanism while retaining backwards compatibility; we want to add a JSON:API-spec compliant, formal translation support.

The current language negotiation mechanism:

• works well for GET requests for individual resources, collection resources and includes.
• does support PATCH requests on translations: updating translations is possible
• does not support  DELETE requests: only the entire resource (entity) can be deleted, not just a translation
• has limited POST support: it does support creating an entity with a non-default langcode in a  POST but does not allow creating additional translations

These are consequences of the peculiarities of the underlying Entity API's translation handling. To follow development of complete and formal support for translations, see: #2794431: [META] Formalize translations support