I live in Berlin.
First off, in your position, I'd look at the location of the language and sports courses, and choose somewhere which had good transport connections relative to those.
In general, U-Bahns run more frequently than S-Bahns, so I'd choose somewhere close to one of those.
On safety:
Basically, Berlin can sometimes be considered "gritty". Lots of tourists come here to drink and go clubbing, and being on public transport late on a Friday or Saturday night, or early on Saturday or Sunday morning can be a bit unpleasant. The U1 subway line at those times can be particularly nasty as people use it to go to clubs like Berghain and Matrix (both near Warschauer Strasse) and leave them utterly drunk and/or high. So you will have people who are comatose or throwing up.
My husband also has to travel on the Ringbahn (part of the S-Bahn) extremely early on Saturday mornings, and said it is unpleasant for the same reasons.
Do note that, in both the Ringbahn or the U1, if you travel later in the day, there is usually no problem at all. What I said does not mean you should avoid all travel on them!
Apart from avoiding the major clubbing areas, the only place I would consider truly unsafe is Marzahn (which you wouldn't be staying in anyway as it's too out of the way).
Kreuzberg and Friedrichshain can have rather too many people drinking or partying late at night, so can get rather loud, and the U-Bahn stations at Kottbusser Tor and Goerlitzer park are renowned for drug dealing. Nollendorfplatz is the center for gay nightlife and has lots of restaurants and clubs, so can get rather loud, but is quieter as soon as you are away from the major streets/squares.
Charlottenburg, Wilmersdorf and Mitte are all pretty central and relatively peaceful areas.
Berlin is loaded with parks. Tiergarten in the middle, of course, but lots and lots all over the place otherwise too. Often there are lakes or underground rivers which meant that an area couldn't be built on.
The only one to REALLY avoid is Gorlitzer park because of the drug dealing.