Add bilingual CLI language support

[Neo-Isshin]

What changed

• Add automatic Chinese/English CLI language detection with English fallback
• Add a persistent language selector to cld config
• Translate core commands, configuration, updates, key handling, and common diagnostics
• Document language selection in both READMEs
• Add coverage for locale detection, persisted preferences, menus, and English output

Impact

MuxLM now follows Chinese and English system locales automatically. Unsupported locales use English, while users can override the choice through cld config or MUXLM_LANG.

Validation

• go test ./...
• go vet ./...
• go test -race ./...
• Manual cld def and cdx def passthrough checks
• Manual Chinese, English, and unsupported-locale output checks