The official language of Poland is Polish.
Non-Polish speakers will find that most of the younger generation (35 and under), speak, or at least understand, English reasonably well. Since English is taught at a very young age (some start as early as 4 years old), only Poles who grow up in isolated towns or communities will not be given English lessons. Older Poles, however, especially those outside the main cities, will speak little or no English at all. Most probably, they will know German or Russian.
A few phrases go a long way in Poland. Contrary to some other tourist cities where natives will often scoff at how bad a foreigner’s use of the native language is, Polish people generally love it when foreigners learn Polish, even if it’s only a few phrases. Younger Poles, however, will jump at the chance to practise their English as well.
Do your homework and try to learn how to pronounce the names of places. Polish has a very regular pronunciation, so this shouldn’t be a problem. Although there are a few sounds unknown to most English speakers, mastering every phoneme is not required to achieve intelligibility. It’s rather about catching the spirit: practice asking for driving directions to Szczebrzeszyn.
Poland’s history has made it a very homogenous society. Outside of the very, very touristy areas of the major cities, you’ll find that there are few, if any, foreigners. Most of the immigrants in Poland (notably Ukranians and Vietnamese) stay in the major cities for work. Be advised that if you are heard speaking English in a public setting you will get looks; many people will listen in to practice their understanding of English. However, speaking English loudly in public also marks you immediately as a tourist, which can be dangerous in certain areas of the cities. Use common sense!