Hi Mirco :)
I know this question is quite old, but I think it's still an important topic.
If you are looking for free German courses in Berlin you can go to Volkshochschule (VHS). All of them offer free courses for all (!) refugees that are either still in the process or that have been rejected. And of course they have regular integration courses for people with residence permission as well ;)
I will upload a leaflet in four different languages for more detailed information about the courses at VHS Neukölln. If you live in other parts of Berlin you can go to their VHS as well to sign up for the classes.
All the best,