I would highly recommend learning some German. I went to Uni in Germany and though most Germans speak English, they greatly appreciate Americans who try. Of course, it's not a language you can learn overnight but even just a few simple phrases usually make people happy. As far as conventions go, Dokomi in Düsseldorf is AMAZING. I was very sick when I got there and I still had a great time. The people are wonderful and very nice and there's a fairly large population of Japanese speakers there as well (was great for me because I could practice my Japanese AND German in the same city
) I don't know if I would suggest it if you can't speak German though. It's outside of the main city of Düsseldorf and navigating can be a bit difficult, even with speaking some of the language. Um...there's Chisai Con in Bremen but that's really small, as the name suggests. There's of course, AniMagic but I don't know much about it. I hope that helps somewhat!