I don't know about Poland, i have never been before, but i am from a European county and had visited many of them too with only speaking just average English. You don't need anything other that English and i bet there will be signs everywhere in Polish/English.

You can download a dictionary or translators to learn some basic words just in case but you won't have a problem. I have been to china using my average English and body language and i was just fine, while 95% of the chinese i met didn't know a single word in English, so you will be fine :P