Dan Foy, Maryana, and I spoke on this stuff, and we thought it would be best to see how this works in Wikipedia Zero land, and then if it works well, continue dialog for the mobile web Wikipedia webroot afterward for the non-Wikipedia Zero use case.

Usually we don't like to vary the cache on Accept-Language, but this is one place where we're exploring the concept. It's possible to try to figure out the language prefix in Varnish, but for now we're trying this as a pure MediaWiki thing.

-Adam