Move language selection to a dedicated page?

I have been browsing various documentation sites, as part of exploration for #1, and I quite like how React has handled their presentation for the documentation translations: https://reactjs.org/languages

By having a separate page, the page can now contain:

• Clear indicators for the "state" / "completeness" of the translation efforts, like "In Progress" and "Needs Contributors".
• Clear links to contribute to each of the language-specific translation efforts.
• Callout to a documentation page, for folks interested in contributing additional languages.
• A lot more space to present translations across many languages. :)

I think all of these are basically non-feasible for a dropdown-based language selector like we have today on docs.python.org.

检查当前基于下拉菜单的语言选择器，并将其与 issue 中链接的 React 语言页面进行比较。定义专用页面的翻译状态指示器、贡献链接以及添加语言的指南；当这些需求能够以多种语言清晰呈现时，即视为完成。