Multi-language interface is unavailable on public proxys

The configuration of language is stored in `zeronet.conf` so user can't change it. All users can only use the language which public proxy owner configured.
Allow users change language simply is not feasible because all users will be affected. So it needs a separate setting form uesrs store in cookie/localstorage and auto-detect via browser's language setting.

该 issue 未列出文件、测试或入口点。首先定位 public-proxy 界面的语言配置及其对 zeronet.conf 的处理方式；然后追踪浏览器语言检测和客户端设置可以集成到哪里。完成的标准是每个用户都可以选择语言或接收浏览器检测到的语言，而无需更改代理范围的设置，并且该选择按用户持久化保存。

由索引模型根据 Issue 内容生成。