I think that the fewer the number of speakers of a particular language, the more likely are they to have a high proficiency of English. I don't know any Polish people, but where I live, in the Netherlands, people speak fairly good English. Most of them have quite a thick "foreign" accent, and most people, even academics make quite a number of mistakes, pronunciation wise, but also grammar wise. On the whole, it's quite decent, compared to let's say native speakers of Japanese, Chinese or French, who, for the most part, speak very little English.
Edit: I actually know one Pole (not personally), Successful English learners: Tomasz P. Szynalski | Antimoon. His American accent is flawless.