In Mozilla, the preferred language is set in Edit->Preferences->Navigator->Languages
If "de" is the first entry in the list of languages there, many sites are displayed in German.
You can test it at the site
http://ibz.ahicre.de
This site is available in four languages: German (de), Romanian (ro), Hungarian (hu) and English (en). If e.g. Romanian (ro) is the first language in your preferences list, you will be redirected to index.html.ro when requesting
http://ibz.ahicre.de
Google also has this feature: when I go to "www.google.com" in IE I always get a German page, in Kmeleon I get the English page.
Christoph