fix: i18n requests locale files that don't exist on disk

[xtantaudio]

Problem

The browser's reported locale (e.g. en-US) is interpolated directly into the translation file request path, but the shipped locale files only exist under the base language code (en), not the full regional variant. Every page load produces 11 404s for missing translation JSON files.

Fix

Normalized the requested locale to match what's actually shipped before the i18next backend fetches it.

Verification

Verified page load now produces zero 404 requests for locale files, tested against multiple browser-reported locale variants.

Summary by CodeRabbit

• Bug Fixes

  • Improved language detection for regional locale codes, ensuring they resolve to the correct available language.
  • Added reliable fallback to English for unsupported or unavailable languages.
  • Prevented missing translation errors for browser locales such as en-US.

• Tests

  • Added coverage for locale mapping, fallback behavior, and language picker resolution across supported locales.

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[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 Plus

Run ID: c2bdedea-d8be-44b5-8028-07e7d3df7202

📥 Commits

Reviewing files that changed from the base of the PR and between cffd35f and e834c49.

📒 Files selected for processing (4)

• apps/web/src/core/hooks/useLang.test.ts
• apps/web/src/core/hooks/useLang.ts
• apps/web/src/i18n-config.test.ts
• apps/web/src/i18n-config.ts

Included review availability: Your plan provides up to 4 included reviews per hour; 0 remain after this review.

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📝 Walkthrough

Walkthrough

The PR derives i18n fallbacks from shipped locale folders, centralizes initialization options, validates locale loading against disk files, and updates useLang to resolve regional and unsupported language codes.

Changes

Locale resolution

Layer / File(s)	Summary
Shipped locale contract and initialization apps/web/src/i18n-config.ts	The configuration exports shipped locales, locale parsing, dynamic fallback construction, load path, namespaces, and shared i18next options. Initialization uses these options and disables explicit regional locale requests.
Locale loading validation apps/web/src/i18n-config.test.ts	Tests compare configured locales with disk folders and validate fallback chains, namespace loading, resolved languages, translation loading, and detected-language mappings without 404 responses.
Language picker resolution apps/web/src/core/hooks/useLang.ts, apps/web/src/core/hooks/useLang.test.ts	useLang matches exact and regional language codes using resolvedLanguage or language, then falls back to English. Tests cover supported regional codes and th-TH fallback behavior.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to e834c

The change normalizes browser locale variants to the shipped translation folders, preventing failed locale requests without changing broader application behavior. No actionable merge-blocking risk remains beyond normal checks and review.

Poem

A rabbit checks each locale trail

English waits when others fail
Regional paths now point just right
Tests guard every loading flight
The picker hops to codes in sight

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly states the main change: preventing i18n requests for locale files that do not exist on disk.
Description check	✅ Passed	The description explains the problem, the fix, and the verification results. It omits the template headings for Changes Made and Checklist, but the required technical information is present.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

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