What i think is you should not use forum sites country wise, yes i know you may get some language problem but that can be resolved by using translator . You should use international forum posting sites .Basically you should think to use those websites which have high PR else they have good traffic on them.
I disagree, if his pages are in German then he should try to get German language links for SEO. Try http://www.abakus-internet-marketing.de/foren/