I wouldn't say that German fanfic generally has a lower quality. For one, I think it depends on the fandom (don't know much about XF). And on the age of the writers and how prevalent beta-reading is. I also often only find interesting English fanfic via reclists nowadays, which is a problem because I don't know of any German reclists that would help me find good German fic.

And I think I'm even more picky when it comes to German fic. Not so much when it comes to the story, it's the style I'm really concious about, and many writers just hate it when a beta tells them that their style could be better.