Add Dutch, Polish, Simplified Chinese, and Traditional Chinese picker translations

[DeBaschdi]

Summary

• Add complete Dutch (nl) and Polish (pl) translations for the VFS picker.
• Add complete Simplified Chinese (zh_CN) and Traditional Chinese (zh_TW) translations.
• Resolve region-specific locales before falling back to the base language and finally English.

Reason

The VFS picker currently falls back to English for Dutch, Polish, and Chinese Thunderbird installations.

Chinese also requires region-aware locale selection. The previous language-only lookup reduced both zh-CN and zh-TW to zh, so it could not select the correct regional translation.

Existing language fallback remains unchanged. For example, pt-BR continues to use the existing pt translation when no regional catalog exists.

Validation

• Parsed all added locale files as JSON.
• Compared all catalogs with en.json: 96 matching keys per locale.
• Checked $1 placeholder parity.
• Verified locale selection for:
  • zh-CN → zh_CN
  • zh-TW → zh_TW
  • nl-NL → nl
  • pl-PL → pl
  • pt-BR → pt
  • unsupported locales → en
• Verified picker.mjs syntax with Node.js.