First and foremost, Turkish. Turkish is the official language of the city, and the days are long gone when Constantinople was a polyglot paradise. Outside of university campuses, Beyoglu, Kadikoy, or Sultanahmet, don't expect to hear much English, and even there, it generally won't be more than very basic. You'll find that Turkish has a lot of French loanwords and many wealthy Turks will speak French. German is also present although again, don't count on it. In the old city around the Aksaray area, expect to hear more Arabic and Russian.
Basically, although you can make yourself understood in most tourist areas, without some Turkish, it will be hard to communicate in the city as a whole.