feat(i18n): add localization support for 13 languages

[therealbrad]

Description

Expands UI language support from 3 languages (English, Spanish, French) to 13, adding German, Italian, Dutch, Polish, Portuguese (Brazil), Vietnamese, Chinese Simplified, Chinese Traditional, Japanese, and Korean.

Introduces a single source of truth for supported locales so adding a new language in the future requires changes to exactly two files (i18n/navigation.ts and i18n/dateFnsLocales.ts) plus running pnpm generate:dev and pnpm crowdin:sync.

Key changes:

• i18n/navigation.ts — canonical locale list and display names (endonym order, CJK at end)
• i18n/dateFnsLocales.ts — date-fns locale map with coverage guard test
• next.config.mjs converted to next.config.ts to import from TS source
• schema.zmodel / prisma/schema.prisma — 10 new Locale enum values
• crowdin.yml — all 12 non-English language mappings configured
• lib/server-date-formatter.ts — locale-specific "at" word for datetime strings
• components/onboarding/InitialPreferencesDialog.tsx — fixed: locale change now sets NEXT_LOCALE cookie and reloads so the language switch takes effect immediately
• Fixed TestPlanIt brand name casing across all locale message files (was being mangled by machine translation)
• Docs and blog post updated

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement

How Has This Been Tested?

• Unit tests — i18n/dateFnsLocales.test.ts includes a coverage guard that fails if a locale is added to navigation.ts without a date-fns mapping
• Manual testing — switched between all 13 languages via Profile > Preferences; verified locale change in initial preferences dialog takes effect on reload

Test Configuration:

• OS: macOS
• Browser: Chrome
• Node version: 24

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• I have signed the CLA (https://testplanit.com/cla)

Additional Notes

Translations are managed via Crowdin. The TestPlanIt brand name has been added to the Crowdin glossary as Do Not Translate to prevent machine translation from altering it in future syncs.

To add a new language in the future:

1. Add the locale to i18n/navigation.ts (locale array + display name)
2. Add the date-fns mapping to i18n/dateFnsLocales.ts
3. Run pnpm generate:dev
4. Add the language in Crowdin and run pnpm crowdin:sync

[therealbrad]

🎉 This PR is included in version 0.27.0 🎉

The release is available on GitHub release

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