If English is the only language you know and you are going to Rio de Janeiro, you would naturally like to know if that will cause a lot of problems. The short answer is yes. Whether that is detrimental or not depends on your need to communicate with the locals when visiting a new country. The service personell in the hotels will be able to inform you in English. Outside of the tourist areas, the people will not be able to speak English. For your visit it will not be an overall problem. A bigger issue is whether being a gringo in general is a problem. As a gringo that speaks Portuguese with a heavy accent - this is always a recurring problem.
Another site - Ingles Porto Alegre.