Sync the de-DE Installation language file.

@infograf768 @sisko1990 This fix 2 issues based on:
https://github.com/joomlagerman/joomla/blob/master/installation/language/de-DE/de-DE.ini

Fixed Issues:
https://github.com/joomlagerman/joomla/issues/28
https://github.com/joomlagerman/joomla/issues/29