Localize a JSON API response

Assume you already return JSON in one language (say English). Product wants fr. You do not want a string catalog or a parallel i18n CMS.

Pattern

1. Build the source response as you do today.
2. Ensure it under your real route identity + target lang.
3. On later requests for that locale, get (or ensure again if you still have the body).
4. On GET 404, fall back to source JSON.

Client asks for fr
  → GET translations?endpoint_url=…&lang=fr
  → 200 localized JSON
  → or 404 → serve source / ensure then serve

Example shape

Source:

{
  "id": "m1",
  "title": "Lunch",
  "items": [{ "name": "Soup", "sku": "SP01" }]
}

Ensure with skips so identifiers stay stable:

{
  "endpoint_url": "https://api.acme.com/v1/menus",
  "lang": "fr",
  "source_lang": "en",
  "data": { "...": "..." },
  "skip_paths": ["id", "items.*.sku"]
}

Same structure comes back; only eligible strings change.

Where to put the call

Common choices:

• Write / publish path — ensure when content changes (menus, CMS publish)
• Read middleware — ensure on first locale miss, get thereafter
• Agent / vibe loop — curl ensure once, then hit your API with Accept-Language / ?lang=

See Quickstart for curl, and Concepts for TTL and siblings when source changes.