That's what Accept-Language[1] is for. They can just serve the right version of the page based on its value. And yes, I know it's annoying when sites just assume you speak the browser language but remember this would just set the default one in the text box, nothing else. Presumably, if you're using a browser in English you can also recognize the word "English" and change it to something else.
Not everyone accesses the internet on their own computer.
Seriously though, I never see the wikipedia front page. I either go through a link in google, start with 'en.' in the address bar, or start typing 'wiki' and choose a previous link. All of these are easier and more portable across browsers and OSs than the knowledge required to change your language locale.
[1]: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14...