JSON:API Multilingual

Drupal Core's JSON:API module serves the language-negotiated translation of an entity and gives you no way to read, create, update, or delete one specific translation on its own. JSON:API Multilingual adds that: per-translation GET, POST, PATCH, and DELETE over the existing JSON:API resource URLs, with a stable HTTP contract that does not depend on Drupal's URL language negotiation.

Selecting a translation

The target language is chosen per request, never by URL path prefix. Use the langCode query string parameter. It is the recommended mechanism for both reads and writes: explicit, cacheable, and accepted on every operation.

?langCode=fr

On reads, add includeFallback=1 to resolve the request through the site's language fallback chain instead of failing strictly (fallback-policy modules such as Language Hierarchy apply automatically; the entity's default translation is the terminal candidate, so an existing entity always returns 200):

?langCode=fr&includeFallback=1

Usage

Read a specific translation (404 if it does not exist):

GET /jsonapi/node/article/{uuid}?langCode=fr

Read with server-side fallback (the language actually served is reported in the Content-Language response header and the langcode attribute):

GET /jsonapi/node/article/{uuid}?langCode=fr&includeFallback=1

Create a new translation of an existing entity (409 if it already exists; only translatable fields may be sent):

POST /jsonapi/node/article/{uuid}?langCode=fr
Content-Type: application/vnd.api+json

{"data":{"type":"node--article","id":"{uuid}","attributes":{"langcode":"fr","title":"..."}}}

Update a single translation (only that translation changes; 404 if it does not exist, use POST to create):

PATCH /jsonapi/node/article/{uuid}?langCode=fr

Delete a single translation (400 for the default translation; DELETE without a language deletes the whole entity):

DELETE /jsonapi/node/article/{uuid}?langCode=fr

Writes (POST, PATCH, DELETE) require JSON:API to be in read-write mode (jsonapi.settings:read_only = false), exactly as for core JSON:API. In read-only mode, writes return 405 Method Not Allowed; reads keep working.

Integrates with

• JSON:API Menu Items (1.2.8+): its rendered menu-tree endpoint honors the same langCode / includeFallback selection, applied per menu link.
• Next.js (next_jsonapi) and Decoupled Router: both extend the same JSON:API controller, and this module composes with them rather than clobbering their behavior.