You'll be fine. Especially young people or people in tourism-related jobs speak good English. Public transport is well-organized and easy to navigate. 

It might help you to plan your trips and tickets ahead of time (i.e. where you are changing subway lines and what ticket you need - they have a rather complicated zones system so you might just want to get a weekly/monthly ticket for the central area once you're in town). Announcements on some subway lines are bilingual in German/English but in general you can just read the station names off the wall in stations and compare to a line map to see when you have to get off the subway. Also when arriving at the airport there is English-speaking staff where you buy the tickets that is able to assist you and so will be your hotel/employer/friendly strangers.