Hi,
Then you can either set the interface to English (seperate from wgLanguageCode) in Special:Preferences to English, or alternatively if you want all users to have this preference set to "English" by default, add the following:
$wgDefaultUserOptions['language'] = 'en';
I haven't tested this, but in theory it should make the wiki behave German with a -by default- English user interface.
Note that this means all anonymous users will have a permanent English user interface, and users with accounts can set it to German if they prefer from Special:Preferences (which overrides $wgDefaultUserOptions ..)
That worked and is exactly what I wanted :)
Thanks a lot
Sigbert