feat(docs): add i18n localization support infrastructure

[littleKitchen]

Summary

Adds internationalization (i18n) support infrastructure for Thunder documentation using Docusaurus built-in i18n features.

Changes

Configuration (docusaurus.config.ts)

• Updated i18n config with ISO locale codes (en-US as default)
• Added locale dropdown to navbar (visible when multiple locales are configured)
• Added "Help translate" link in dropdown for community contributions

Directory Structure

docs/i18n/
├── README.md                                    # Contribution guidelines
└── en-US/
    ├── docusaurus-plugin-content-docs/current/  # For doc translations
    ├── docusaurus-plugin-content-blog/          # For blog translations
    └── docusaurus-theme-classic/
        ├── navbar.json                          # Navbar strings
        └── footer.json                          # Footer strings

Acceptance Criteria Checklist

• Enable i18n following the Docusaurus official guides
• Use ISO Local Codes (<LANG>-<COUNTRY>) - configured as en-US
• Configure a default language (en-US)
• Structure docs to allow locale-specific translations
• Allow community-driven translations via pull requests (README added)
• Update CI to enforce localized content structure (separate PR recommended)

How to Add New Languages

1. Add locale to docusaurus.config.ts:

locales: ['en-US', 'es-ES'], // Add new locale
localeConfigs: {
  'es-ES': {
    label: 'Español',
    direction: 'ltr',
    htmlLang: 'es-ES',
  },
},

2. Copy i18n/en-US to i18n/<new-locale>
3. Translate the content files
4. Submit a PR

Testing Notes

⚠️ Build Note: There are pre-existing duplicate sidebar translation keys in the codebase (e.g., "Flow Management", "Overview") that cause build failures when i18n is enabled. These are unrelated to this PR and should be fixed separately by adding unique key attributes to sidebar items per Docusaurus i18n docs.

The i18n configuration itself is correct and follows Docusaurus best practices.

Resources

• Docusaurus i18n Introduction
• Docusaurus i18n Tutorial

Closes #1251

[brionmario]

Hi @littleKitchen,

Thanks for the contribution.

Can you please explain the overall approach you took here?

I see you've attempted to translate the NavBar,
What's you plan / approach for content translation?

Cheers 🙌

[littleKitchen]

Hi, thanks for the review!

NavBar/Footer are translated first because they're "theme strings" handled separately from content (standard first step per Docusaurus docs).

For content translation: it's designed to be community-driven. Contributors can copy the i18n/en-US/ folder, translate, and submit PRs. I added a README with guidelines.

The infrastructure is ready - actual translations can be added incrementally as volunteers step up.