Update language translations

Update language translations

Write draft values for the language. The body names the keys to change and the value to give each of them; a key the body does not name keeps the value it has, and `draft_value: null` clears one back to blank. Nothing a shopper sees changes: publishing the language is what promotes draft values to published copy. Only `draft_value` is writable. Every other property the read publishes is server-owned — the catalogue supplies the key, the group, whether it is required, and the source copy; a publish supplies the published values; the sync supplies the staleness — so naming one is a 422 pointing at it rather than a value silently dropped. A key may appear once; a second entry for the same key is refused rather than resolved. A 422 problem with code `invalid_translation_key` means the body names keys this language cannot accept, with a pointer per offending entry and a `reason` of `unknown` (the catalogue has never carried it) or `retired` (the copy it translated is gone). A 422 `validation_failed` with a pointer at `/translations/{index}/draft_value` means a value dropped a placeholder its source copy requires — a value that loses `{price}` would render a broken sentence in the storefront. The write clears the customiser cached storefront configuration and runs the advisory language sync. It never regenerates your Shopify products, product images, or product cache. Send an `Idempotency-Key` header (1-255 visible ASCII characters, e.g. a UUID) to make the write safe to retry: repeating the same key with the same body replays the original response (marked `Idempotency-Replay: true`) without executing again; the same key with a different body is a 409 problem with code `idempotency_key_conflict`. The header is optional here, as on every update operation. Requires the `customisers:write` scope. A 400 problem means the body is not valid JSON or the `Idempotency-Key` header is malformed; a 422 problem with code `unknown_field` means the body names properties this operation does not define. 401, 403, 404, and 429 problems behave as on the read operation, with `customisers:write` as the required scope.