[nando2002]

Why you just don't go to the shop where you bought the game and exchange it for the German version?

aside that...
to make the german language appear in the language selection screen, in the support folder there's a "localization.ini" file. open it with notepad and add the following line to [LOCALES] group: de=1

this will make the deutsch language appears in the language selection screen and the installation will show everything in german but won't install because it miss the transforms for german language.

edit: I've created a faked transform (mst) for the german language using the english one.
the german files must be renamed to the english ones and overwritten inside the cab. setup will install them.