A good idea is to advertise sites that have the actual billing set up for that specific country - if it's in the language of the surfer even better - be careful though... a bad translation is worse than none at all.
To use GeoIP for including the correct advertising in your sites is a must, the best idea is to promote sites that actually are from affiliate programs and people that live in this country and use this language.
That said... if you have german-speaking traffic, you should talk to me to maximize your income. If you don't like to watch ratios or wait for results etc. you may also just sell me your german-speaking (AT,DE,CH) traffic
ch33rs,
Steve / JOKER