Backend/i18n: Localize region & spoken language names

[tristanlabelle]

Updates the GetLanguages and GetRegions APIs to return names localized in the current user's locale, at least where the Unicode CLDR defines those translations, which should hold for all regions and most common languages.

In both cases, our database stores 3-char codes, but the Unicode CLDR lookup is by 2-char codes. I had to add a mapping from 3-char codes to 2-char codes (where it exists) as to be able to leverage the CLDR without changing our database tables. In the case of languages, I updated languages.json to add 2-char codes for languages that have one.

In the end this fix is independent from #9169, but that frontend change is still an improvement.

Fixes #8424

Testing

Ran local backend + frontend, added tests.

Backend checklist

• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

[CouchersBot]

Mobile

Install the Dev Tool (iOS via TestFlight, iOS Simulator, or Android .apk) here.

Scan the QR with your phone camera, or tap Open in Dev Tool on the device, to open this branch in the installed Dev Tool dev client.

iOS	Android
Open in Dev Tool	Open in Dev Tool

Deep links

iOS

couchers-devtool://expo-development-client/?url=https%3A%2F%2Ff5ebb4db--ota.preview.couchershq.org%2Fios%2Fmanifest

Android

couchers-devtool://expo-development-client/?url=https%3A%2F%2Ff5ebb4db--ota.preview.couchershq.org%2Fandroid%2Fmanifest

Web (Vercel)

View the Vercel web preview for this branch.

Backend

Schema	Schema diff	Sample emails

Other

Protos	Backend coverage	Web coverage

[nabramow]

@tristanlabelle Looks like some web tests are failing!

[tristanlabelle]

@nabramow , the failures were a fluke / unrelated flakiness. I rebased and tests passed. Ready for ✅

FAIL features/publicTrips/PublicTripsSection.test.tsx (26.293 s)
● PublicTripsSection › create › submits valid form data to createPublicTrip and closes the dialog
thrown: "Exceeded timeout of 10000 ms for a test.

[aapeliv]

Babel claims to support RFC3066, which claims to support also the three character codes https://datatracker.ietf.org/doc/html/rfc3066.html#section-2.2

This appears to work for me:

$ uv run --with babel python
Python 3.14.2 (main, Dec  5 2025, 21:11:58) [Clang 21.1.4 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import babel
>>> babel.Locale.parse("fin").get_language_name("swe")
'finska'
>>>

do we then not need modification to the languages source of truth file?

[tristanlabelle]

@aapeliv Thanks for catching the support for 3-letter language codes. I'll remove the extra complexity. We do need the 2-letter code for regions though.

[tristanlabelle]

@aapeliv 2-letter language codes are out. Ready for ✅

[aapeliv]

sorry one more thing on the backend side!

[tristanlabelle]

@aapeliv , should be good now!

[aapeliv]

Lgtm!

[github-actions]

📝 Release Notes

This PR should be included in the release notes.

Suggested release note:

Localized region and spoken language names across the app so they appear in your selected language where translations are available

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: include
Reasoning: This fixes a visible internationalization issue by showing region and spoken language names in the user's selected language instead of always in English, which is a noticeable improvement for non-English users editing/viewing profile-related fields.