fix(i18n): add 6 missing translation keys and enforce locale parity in tests

[Zuhef]

Fixes #2993

Both halves of the issue

1. The six missing keys. Added to the locales that lacked them:

Locale	Key added
ar-SA, da-DK	chat.favorites.remove
de-DE, it, ko	components.DatePickerInput
ja	chat.fileUpload.browse

2. A parity test over the real locale files - the durable half. backend/tests/test_translations.py now has a TestShippedTranslationFileParity suite that walks backend/chainlit/translations/ and compares every file against en-US.json using the project's own compare_json_structures.

It is parametrized per locale, so a regression names the offending file in the test id and lists the exact keys in the assertion message:

FAILED backend/tests/test_translations.py::TestShippedTranslationFileParity::test_locale_matches_ground_truth_structure[da-DK.json]
AssertionError: da-DK.json differs from en-US.json:
    Missing key: 'chat.favorites.remove'

Two details from the issue notes:

• en-US.json needs no special-casing - it is included in the parametrization and comparing it to itself is trivially clean.
• Because the locale list is globbed at collection time, an empty glob would make every parametrized case silently vanish rather than fail. test_translations_directory_is_discoverable guards that: it asserts the directory exists, that en-US.json is in it, and that more than one locale file was found.

Verification

Ran against the local source tree (PYTHONPATH=backend):

• pytest backend/tests/test_translations.py - 81 passed (35 pre-existing, plus 46 parametrized locale cases and the discoverability guard).
• Independently confirmed the six gaps before the change, and zero after, by running compare_json_structures over all 23 locale files.
• Mutation-checked the new test rather than trusting it: deleting chat.favorites.remove from da-DK.json produces exactly the failure quoted above; restoring it returns to 81 passed. A parity test that cannot fail would be worse than none.

On the translations themselves

The issue notes the six strings are best done by native speakers, and I am not one for these locales. Rather than inventing phrasing, each follows terminology already present in the same file or the established house style:

• ar-SA reuses the "remove" wording from its existing chat.fileUpload.actions.removeAttachment.
• da-DK uses Fjern, matching its own removeAttachment value.
• ja follows the same pattern as its existing dragDrop and removeAttachment entries.
• The DatePickerInput strings follow the existing fr-FR / es / nl phrasing ("Choisir une date" / "Elige una fecha" / "Kies een datum").

I would welcome native-speaker review on all six, and I am happy to take corrections in this PR. The parity test is the part I would defend on engineering grounds; the wording I would defer on.

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Summary by cubic

Adds six missing i18n keys across shipped locales and introduces a parity test to ensure all locale files match en-US.json. Prevents missing-key renders and blocks future gaps.

• Bug Fixes

  • ar-SA, da-DK: add chat.favorites.remove.
  • de-DE, it, ko: add components.DatePickerInput placeholders (single, range).
  • ja: add chat.fileUpload.browse.

• Tests

  • Parametrized parity test compares each shipped locale to en-US.json using compare_json_structures, failing with exact missing keys.
  • Add a discoverability guard so tests don’t pass vacuously if the translations directory changes.

^{Written for commit 1bc1a30. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files

_{Re-trigger cubic}