Thomas-
## If you want a non-English wiki, add a line like this $wgLanguageCode = "de";
in LocalSettings.php. But the "Startpage" still appears in english.
If you have changed the settings after the install, then the MediaWiki namespace is still in the language that was used during the install. You can verify this by setting
$wgUseDatabaseMessages=false;
in LocalSettings.php. If, after clearing your cache, the wiki is now in German, the problem is that your database contains the English messages.
If you don't need the MediaWiki namespace, you can leave it at that. If you do need it, I suggest reinstalling the wiki, but setting $wgLanguageCode before doing so.
Regards,
Erik