Dear Steven..
I found the solution...
http://communities.ptc.com/community/creo/blog/2011/11/28/language-change-back-to-english
I didin't understand how could I modify the file parametric.psf.
But is simple you have to open it with the notepad and just add ENV=PRO_LANG=english at the end.
Probably you have to replace english with german..
Bye
Guido