Once a page is requested the browser send a http request to a webserver. With that it send which browser and operating system you are using. I believe it sends your operating system's / browser's language too. The part that information is send in is the HTTP Header. I have more information about it at home, but I am at work now, ain't got it here. Wont be at home for a lot of hours
Search some script to get info out of the http headers.
Good luck,
let me know what you have found.
Robert Jansen