Fill in missing translation keys and add a parity test

[sridhar-3009]

Closes #2993.

Six keys present in en-US.json were missing from other locale files under backend/chainlit/translations/:

Locale	Missing key
ar-SA	chat.favorites.remove
da-DK	chat.favorites.remove
de-DE	components.DatePickerInput
it	components.DatePickerInput
ko	components.DatePickerInput
ja	chat.fileUpload.browse

Filled in all six (translations below, happy to have a native speaker double check ar-SA/da-DK/de-DE/it/ko/ja):

• ar-SA/da-DK: "Remove favorite"
• de-DE/it/ko: the DatePickerInput single/range placeholders
• ja: "Browse Files"

The more important part is why this keeps happening: chainlit lint-translations only checks the consuming app's .chainlit/translations/ directory (per lint_translations() in config.py), never the package's own backend/chainlit/translations/, and no CI workflow touches translations at all. So a PR that adds a key to en-US.json and forgets the other files goes green, which is exactly what happened three times here (#2654, #2792, and one predating both).

Added TestTranslationFileParity to backend/tests/test_translations.py, which loads the real backend/chainlit/translations/ directory (via TRANSLATIONS_DIR from chainlit.config) and asserts every locale file has every key en-US.json has, parametrized per file so a failure names the exact locale. en-US.json compared against itself is included and passes trivially, as expected.

All 57 tests in the file pass, including the 23 new parametrized cases. Ran scripts/lint.py, scripts/format.py --check, and scripts/type_check.py clean.

---

Summary by cubic

Filled missing translation keys across six locales and strengthened the parity test against en-US.json to also catch structure mismatches. This keeps all shipped locales complete and consistent.

• Bug Fixes
  • Filled keys:
    • ar-SA, da-DK: chat.favorites.remove
    • de-DE, it, ko: components.DatePickerInput.placeholder.{single,range}
    • ja: chat.fileUpload.browse
  • Added TestTranslationFileParity in backend/tests/test_translations.py that validates every locale includes all keys and correct shapes from en-US.json (extra keys allowed).

^{Written for commit 0c59b70. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 7 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[sridhar-3009]

Good catch — fixed. The test now fails on any non-"Extra" divergence (missing keys and structure mismatches alike), not just missing keys. Pushed.