From the moment you land until taking off to come back to the United States you will likely find that 75% of the people speak some English but almost all can understand what you are saying. I would say that 90% speak good English in the tourist areas. This is very different than other places I have visited like Mexico and the Dominican Republic. In the tourist towns of Jaco, Manuel Antonio and Quepos you are going to find that everyone understands English and most speak it well. In fact, every single restaurant I went to had an English menu and a Spanish menu.
If you trek into some of the me remote parts of Costa Rica I am certain that English is not as common but if you stay close to the tourist areas, you will have no trouble communicating. I have very little Spanish knowledge and I never once felt as if I could not communicate with the locals.