feat(i18n): add Luxembourgish language support

[0xSysR3ll]

Description

This PR adds the necessary keys to support Luxembourgish language.

• Fixes Add Luxembourgish language #2421

How Has This Been Tested?

Screenshots / Logs (if applicable)

Checklist:

• I have read and followed the contribution guidelines.
• Disclosed any use of AI (see our policy)
• I have updated the documentation accordingly.
• All new and existing tests passed.
• Successful build pnpm build
• Translation keys pnpm i18n:extract
• Database migration (if required)

Summary by CodeRabbit

• New Features
  • Added Luxembourgish (Lëtzebuergesch) language support across the app: selectable UI language, included in available locales, and integrated into locale loading and external language mappings.
  • Locale resources now load for Luxembourgish at startup so translations are applied consistently.
  • External language mapping updated to allow direct conversion for "lb" where applicable.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 80d21141-cab6-4e20-8ad0-14abc65c2333

📥 Commits

Reviewing files that changed from the base of the PR and between 53536bd and 6507baf.

📒 Files selected for processing (4)

• server/api/tvdb/interfaces.ts
• server/types/languages.d.ts
• src/context/LanguageContext.tsx
• src/pages/_app.tsx

🚧 Files skipped from review as they are similar to previous changes (4)

• src/pages/_app.tsx
• server/api/tvdb/interfaces.ts
• server/types/languages.d.ts
• src/context/LanguageContext.tsx

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📝 Walkthrough

Walkthrough

Adds Luxembourgish ('lb') across the app: TVDB language mapping, AvailableLocale type, client language list, and locale loader import/case.

Changes

Cohort / File(s)	Summary
Server language & mappings server/api/tvdb/interfaces.ts, server/types/languages.d.ts	Added lb mapping to TMDB→TVDB (ltz) and added 'lb' to the AvailableLocale union.
Client locale & context src/context/LanguageContext.tsx, src/pages/_app.tsx	Registered lb in availableLanguages as "Lëtzebuergesch" and added lb.json to the locale loader switch.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• feat(i18n): add Vietnamese language support #2670: Similar locale additions—updates mapping, AvailableLocale, availableLanguages, and locale loader for a new language.

Suggested reviewers

• fallenbagel
• M0NsTeRRR

Poem

🐰 I nibble keys and hop through code,
A tiny tongue finds its new road.
'lb' peeks out, soft and bright,
Lëtzebuergesch joins the light.
Hooray — I hopped it into sight!

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title accurately describes the main change: adding Luxembourgish language support with a new language code 'lb'.
Linked Issues check	✅ Passed	The PR implements the core requirement from issue #2421 by adding Luxembourgish language support across server and client files.
Out of Scope Changes check	✅ Passed	All changes are directly related to adding Luxembourgish language support with no extraneous modifications detected.

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📝 Coding Plan for PR comments

• Generate coding plan

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@src/pages/_app.tsx`:
- Around line 68-69: The loader in src/pages/_app.tsx returns
import('../i18n/locale/lb.json') for case 'lb' but the file is missing; add the
missing Luxembourgish message bundle at src/i18n/locale/lb.json (exporting the
same shape as other locale files) or remove/guard the 'lb' branch until you add
it; update the file to include the required translation keys used by the app so
LanguageContext.tsx and server/types/languages.d.ts remain consistent and
SSR/runtime imports succeed.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 177dec37-80ad-4b51-b3ef-ea2fc32d77af

📥 Commits

Reviewing files that changed from the base of the PR and between 4d2b658 and 0b96d37.

📒 Files selected for processing (4)

• server/api/tvdb/interfaces.ts
• server/types/languages.d.ts
• src/context/LanguageContext.tsx
• src/pages/_app.tsx

[fallenbagel]

#2680

Once this is merged, this PR can be merged

[fallenbagel]

@0xSysR3ll could you rebase this pr?