Cannot say for German fandom, but from what I've seen from the early days of Russian online fandom the main reason for translating probably was that the fandom had still been very young and thus not strong enough to immediately start producing something of its own. By translating other's work fans were kind of learning the rules of the game, so to speak. Now, about 10+ years later the percent of fanfiction written originally in Russian has increased compared to the one of translated works.