feat: add 17-language support with auto-detection and language switcher

Supports: en, ja, zh, zh_TW, ko, es, pt, fr, de, it, ru, ar, th, id, tr, nl, vi
- Auto-detects browser language
- Dropdown language switcher in navbar
- Saves preference to localStorage
- RTL support for Arabic

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: KEN0421