Use correct ISO 639-1 language code (vi) instead of country code (vn),
consistent with existing translations (zh-CN, ja, ru).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Add full Vietnamese (Tiếng Việt) translation of README.md and update
language selector links across existing README files.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>