Hi
The selection that you made is only valid on installation.
If you want to use German language you need to get files from joomla.org translation or from
http://www.joomla.de/
This is basic joomla handling and it's valid only for joomla core files.
To have the template translated, after you install German language, go to ../corporate2/language/en-GB/en-GB.tpl_gk_corporate2.ini, copy this file and paste to ../corporate2/language/de-DE/
Them rename this file with your native language prefix, so it should stay like this:
../corporate2/language/de-DE/de-DE.tpl_gk_corporate2.ini
Open file and start making your own translation.
Cheers