Add Language Settings

[sdfwss]

This project has a language choosing module, but it might show wrong default language.
I'd like to add a language setting, which can help users change the language.

The file common/lib/locale.ts does have the Chinese selection, and my system language is Chinese, but DOMD still shows English.

[do-md]

Sounds right. Relying only on locale detection is a bit fragile.

I’ll add a language setting with Follow System / English / 简体中文 / 日本語, and make the manual choice override auto-detection.

Leaving this open until it ships.