[snip] you can use Accept-Language as a starting point but you can't really trust it. You can set UI language based on it, but in that case users should always have an option to easily change it.
Just to be clear -- my idea is to add, not to remove. Currently, there is AFAIK _NO_ possibility at all, how an anonymous user could choose the language of the interface. If you visit Commons, then until logging in, the whole interface is in English, and you have to log in using that English interface.
Logged-in users can choose their language in preferences, and I do not want to change that.
(Note that if Accept-Language would be "totally unacceptable", there just has to be some other way to choose interface language by anonymous users, e.g. commons.wikimedia.org/wiki/Foobar?lang=cs, or whatever.)
-- [[:cs:User:Mormegil | Petr Kadlec ]]