Add a banner allowing the user to redirect to a localized version

[timothyqiu]

The language switcher on the bottom left is hardly discoverable for users not familiar with ReadTheDocs, as it's hidden by default.

I think it would benefit more users if we do some detection and provide the option to redirect to the user's preferred language on the docs page. For example, visiting https://www.adobe.com/ might prompt you with a similar dialog if your browser's preferred language is not English.

I'm not sure if ReadTheDocs/Sphinx actually allows implementing this. But if it does, the logic might be something like this:

1. Check the Accept-Language header to see if we provide a version that matches the user's preferred language.
2. If such a version exists, show a banner (like the "Up to date" banner) that contains:
       * A message possibly in that language.
       * A link that redirects to the same page in that language.
       * A close button that prevents this banner from showing again, at least in this session.

See also godotengine/godot-proposals#10538

[tetrapod00]

I'm in favor of this! One thing to consider is that the banners at the top of the page already take up a large chunk of space, and another banner only adds to that. Quick mockup with inspect element, on my default firefox browser at 100% zoom on a 1080 display:

This is the worst case scenario, with a tall "latest" docs banner and 3 paragraphs in the translation banner.