Regarding to this topic (http://forum.coppermine-gallery.net/index.php/topic,50460.0.html),
'filename_how' => 'Wie soll der Dateiname modifiziert werden',
should be
'filename_how' => 'Wie soll der Dateititel modifiziert werden',
in both german language files.
Has already been fixed in SVN. Thanks for your report.