There’s no direct invariant mapping between languages and geographical areas, so it’s unlikely that any such site exists IMO, just because it’s a hard problem.
Wikipedia is probably one of the most thoroughly internationalized sites on the Web, and also probably a good place to learn about the backgrounds of languages.
When you click on the language links at Mozilla, what happens is that certain files get swapped when the page renders. If parts of the page have been translated into the target language, then the translations will exist in separate files. These are the ones that get “swapped in” when you click the link.
I’m not entirely clear on whether this is actually called Internationalization or Localization – https://en.wikipedia.org/wiki/Internationalization_and_localization has more information.