@simon
In constrast to Accept, I think that Accept-Language doesn't fit reality that well.
Let's assume we are in an ideal world where users can cause their browsers to emit Accept-Language values exactly like they want (in particular different ones for different sites), and UI affordances for temporarily/permanently/... switching that for a given site/... are intuitive for all.
Even in that world Accept-Language is insufficiently expressive. Let's say we have a site with UI translated into many languages and with automated translation for contents. There's no way to express a preference between "English UI, content auto-translated into English" and "English UI, German content in German, other languages auto-translated into English". Also, it's impossible to express different preferences for UI and content.
The situation for any site that offers search over a dataset in multiple languages is even weirder: should Accept-Language cause search to exclude or downrank some results?
Many people (in particular people learning a language) would want to be able to control all of these aspects independently, which wouldn't be possible if everyone took heed of Accept-Language.