Well, I must say, everybody here was afraid of the Croatia-Turkey match for the same reason: There are some 70.000 Turks and 20.000 Croats living in Vienna, plus some 100.000 fans from both sides were expected to visit the city for the game. Vienna has put ca. 1.000 extra officers on duty Friday night, fearing clashes between the fans. But nothing serious happened.
I was watching the game in Ottakring, a district populated mainly with foreignerns, that was proclaimed to be a potential war-zone. But Croatians were quite cool about the outcome of the game. The celebration was huge - like nothing I have ever seen in Vienna - and the traffic chaos that resulted from it was the only actual trouble. A large number of Croatian fans, mainly the ones living in Vienna, were celebrating along with the Turks; many of them exchanged t-shirts, so after midnight you couldn't distinguish anymore who in the crowd belonged to which side. Austrians joined the celebrations as well, and the atmosphere and good vibes that could be felt all around the city last night I will remember for the rest of my life - it was really awesome and a lot of fun, despite the fact I was actually cheering for the Croatian side.
There were minor troubles nearby in Ottakring where first Croatian and then Turkish fans clashed with the police, throwing bottles and stones, but those were caused by a tiny minority of drunk idiots (almost all of them guests - their local colleagues were trying to calm them down) who were quickly arrested. Those clashes have left only two persons slightly injured.
So I think that Berlin shouldn't be panicking - if it worked well in Vienna, it will probably go well in Germany too. I'm sure there will be a few idiots who will be begging to get arrested in the night of the game, but in general I trust the people will take it cool. Except that - Turkey will win... so don't worry.