I was on holiday in Berlin last year. I checked out some classes in the city to keep up with the training.
Most important thing for me was the language thing. My German is prett poor, so I needed a place were they had some English. I could understand the Japanese commands ok though, so that made it a bit easier.
If you have good German though this won't be much of a problem for you. http://www.aikido-verein-hannover.de/english.html
Here is a club that has English translation on its website. They may be worth talking to if language is an issue.
I know Hannover is in a different part of Germany, but for what it is worth, almost everyone in Berlin I meet spoke decent to excellent English. Most European countries are bi-lingual.