Maybe not technically Celts, but can Romans be seen as an 'older' culture than the German ones? And Jews as an 'older' culture.
And it was only recently that I read about the massacre of Jews by Romans.
"According to Cassius Dio, 580,000 Jews were killed, 50 fortified towns and 985 villages razed."
http://en.wikipedia.org/wiki/Simon_bar_Kokhba
And "Germans" can sleep safe - because the center of your culture is now decentralized. Maybe it's in America, maybe in Sweden/Denmark, maybe in Spain. And even Russians have recently rediscovered their Viking roots.
Slavs are also spread wide. And both Germans and Slavs have numerous internecine, deeply dividing conflicts.