Turkish the official language of the country and is spoken everywhere. In the south, you'll hear a lot more Arabic (basically the same as Syrian dialect), sometimes you'll hear Kurdish in the east, but not unless you are looking for it. Although interesting, these languages are mainly community languages and everyone but the eldest people will be fluent in Turkish.
You'll notice that english is not one of those languages. Outside of big cities (and even there it's not too common) you won't find many English speakers. It's best to carry a dictionary or to develop a little bit of turkish language skills (numbers, yes, no etc) before you head out of Istanbul.