Resolve "Clean up existing translations and adding new ones - zh-hant, zh-hans, nl, de, fr, ja"

Closes #277 (closed)

Changes

Translation Fixes

• zh-hans.json: Translated 101 previously untranslated English values
• zh-hant.json: Verified and improved existing translations (fixed "前沿版" → "實驗版", removed duplicate word in RESET)
• nl.json: Complete translation (678 keys) with proper Dutch translations
• de.json: Complete translation (678 keys) with proper German translations
• fr.json: Complete translation (678 keys) with proper French translations
• ja.json: Complete translation (678 keys) with proper Japanese translations

I18N.TITLE Fixes

All language files now use proper language name format:

• en.json: "English (UK)"
• zh-hans.json: "中文简体"
• zh-hant.json: "中文繁體"
• nl.json: "Nederlands (Dutch)"
• de.json: "Deutsch (German)"
• fr.json: "Français (French)"
• ja.json: "日本語 (Japanese)"

Other Changes

• Added demo mode support for nl, de, fr, ja in demo.service.ts
• Added scripts/check_translations.py for validating translation files
• Removed extra DASHBOARD.INFO.VERSION key from zh-hans
• All translation files pass check_translations.py validation

Notes

• Remaining "English" values are proper nouns, technical terms, acronyms, and words identical in both languages (e.g., OK, Kernel, Dashboard, DHCP, ext4, SnakeoilOS)