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
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.