Turkish is the official language of Turkey, but English is widely understood in Istanbul; tourists are frequently startled by the high proportion of English spoken by Turks. An attempt to converse Turkish is appreciated and considered good manners, however doing so would be very kind.
Travellers spend the most of their time in Sultanahmet, Cappadocia, or resort areas on the Mediterranean and Aegean shores. Turks they meet there usually work in tourism, so they understand English as well as a variety of other languages. If you go to non-touristic areas, you might have difficulty in finding someone speaking English, though.