feat(i18n): add Korean locale and graceful fallback for missing locales

[YeonghyeonKO]

Summary

• Add ko.json with full Korean translations for all 388 UI strings
• Register Korean (한국어) in SUPPORTED_LANGUAGES constant
• Add try-catch in loadLanguage() to gracefully fall back to English when a locale file is missing

Problem

When a user's browser language is set to Korean (navigator.language returns "ko"), the app attempts to dynamically import ./locales/ko.json which doesn't exist. This causes an unhandled runtime error:

Uncaught (in promise) Error: Unknown variable dynamic import: ./locales/ko.json

This crashes the app on startup for Korean-speaking users. The same issue affects any browser language not in the current locale set.

Fixes #12735

Changes

File	Change
src/frontend/src/locales/ko.json	Full Korean translations for all UI strings (matching ja.json pattern)
src/frontend/src/constants/languages.ts	Add { code: "ko", label: "한국어" } to SUPPORTED_LANGUAGES
src/frontend/src/i18n.ts	Wrap dynamic import in try-catch to fall back to English for unsupported locales

Related Issues & Commits

• bug(i18n): loadLanguage() crashes on unsupported browser locale (missing fallback) #12735 — loadLanguage() crashes on unsupported browser locale (missing fallback)
• feat: IBM Globalization Pipeline integration and i18n setup #12226 — IBM Globalization Pipeline integration and i18n setup (initial i18n infrastructure)
• chore(i18n): disable automatic browser language detection #12671 — chore(i18n): disable automatic browser language detection
• 99ee7a3 — feat(i18n): lazy-load non-English locale files via Vite dynamic imports

The loadLanguage() fallback fix is complementary to #12671 — even with automatic detection disabled, users who manually select an unsupported language (or have a stale localStorage preference) would still hit this error.

Test Plan

• Set browser language to Korean → verify app loads with Korean UI
• Set browser language to an unsupported locale (e.g., th) → verify app falls back to English without errors
• Verify Korean appears in Settings → Language selector
• Verify all translated strings render correctly in Korean

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Walkthrough

The pull request adds Korean language support to the frontend application by registering the language in the supported languages list, implementing error handling for missing locale files during dynamic import, and providing a comprehensive Korean translation file.

Changes

Cohort / File(s)	Summary
Language Support Configuration src/frontend/src/constants/languages.ts, src/frontend/src/i18n.ts	Added Korean ("ko", "한국어") to supported languages list and wrapped dynamic locale import in try/catch block to gracefully handle missing locale files with warning logging.
Korean Locale Translations src/frontend/src/locales/ko.json	Added complete Korean translation file covering UI text for errors, success states, alerts, dialogs, chat messaging, settings, store browsing, tables, component labels, navigation, modals, and parameterized placeholders.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

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Important

Pre-merge checks failed

Please resolve all errors before merging. Addressing warnings is optional.

❌ Failed checks (1 error, 3 warnings)

Check name	Status	Explanation	Resolution
Test Coverage For New Implementations	❌ Error	The PR adds error-handling for missing locale files but i18n.test.ts lacks tests for the try-catch fallback mechanism.	Add tests verifying non-existent locale loading, console.warn calls, and graceful English fallback behavior.
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Test Quality And Coverage	⚠️ Warning	PR introduces try-catch error handling in loadLanguage() but lacks test coverage for error paths, non-existent locales, and newly added Korean locale.	Add tests for unsupported locale handling, console.warn verification, English fallback behavior, and Korean locale loading to src/frontend/src/i18n.test.ts.
Test File Naming And Structure	⚠️ Warning	PR adds Korean locale support to i18n system without including test files for the new functionality.	Add test files to verify Korean locale loading, fallback mechanism, and language constant exports work correctly.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main changes: adding Korean locale support and implementing error handling for missing locales.
Excessive Mock Usage Warning	✅ Passed	Test file demonstrates appropriate minimal mock usage with jest.spyOn() used twice and immediately restored, testing actual i18n behavior rather than replacing core logic.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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[YeonghyeonKO]

@RamGopalSrikar @Adam-Aghili @vjgit96
Hi, this PR is related to #12226 (#12226) and fixing fallback for missing locales. Could you please review the issue(#12735) and this PR?

[RamGopalSrikar]

#12733 refer to this PR, language for korean not included yet.

[YeonghyeonKO]

@RamGopalSrikar Right, this PR has two features:

• applying fallback mechanism for missing locales (which is an urgent feature, has to be merged)
• adding Korean locale

[YeonghyeonKO]

Superseded by #13674, which is rebased onto current main and covers all 2,228 keys in en.json. Closing this one.