Add optional language selector widget

We're in the process of internationalizing Thimble, but there's currently no user-facing UI for changing the language; the only way to do so is by changing the language setting in the user's browser preferences. However, many times users who prefer a particular language haven't set that preference, so many sites (including Mozilla's own websites) provide a language selection widget somewhere on the page.

It _seems_ like the universal navigation is a good place for this, since it's a feature we want all our properties to support universally, and ideally changing the setting in one place should change it across all our properties. While doing the latter might be technically challenging, it'd be nice to at least have the widget be in a common location across all our properties.

Because of this, I'm filing this ticket here, rather than specifically in the thimble project, but we can mark it as closed if we decide it's not a good idea.

@iamjessklein and @k88hudson, what do you think?

Start by reviewing the universal navigation entry point and how Thimble currently derives language from browser preferences. Clarify whether the widget should apply across properties or only provide a common location, then define the behavior and a way to verify language changes before implementation.

Written by the indexing model from the issue text.