It should be possible to change the language into geman and french. But in the options menue is no such option. Do I have to tweak the ini or something like that?
thx

You probably need to download German or French version - just select your prefered language from dropdown menu.

I just got this solution from the nice gog support :-)

Hi, the game's language settings are located in the installation directory.
By default, this will be:
"C:\GOG Games\Psychonauts\Language Setup.exe"

Make sure to launch language setup as an Administrator, and enjoy the game in German!