Although my course was taught in English, you should definitely learn the language. Even living in the capital city of Warsaw, it is a very lonely existence if you only know English. While everyone under the age of 30 will probably speak English, often when dealing with more important businesses such as the police, local government offices, shops and transport inspectors, you will find that they do not speak English. The Polish are one of the proudest nations in the world and at least trying to engage in their notoriously difficult native language is met with extraordinary respect.