As for the parties, I guess it'll depend. Sometimes, being a foreigner, not understanding the language and all the little social nuances that are common in various cultures you might feel more left out having been invited to the party than staying at home
I guess you're right - If you would be invited as the only non-Pole, you get an isolated feeling anyway: everybody's laughing about something and you don't know the hell they're laughing about. Is it about you or maybe about something that in your culture is perceived as not being funny at all. In all the most likely scenario would be that one would end up with a drink in the corner all alone amongst 50 ppl, with only now and then somebody coming up to you to ask you the same question in poor English everybody else has already asked you. Also it would present a kind of burden on the host/hostess as he or she would feel obliged to spend an uneven amount of time with you as he or she would be the only one that can converse with you about some shared experiences. In all, I do understand that.
Therefore theres probably quite alot of gossip to be enjoyed.
But that is weird: if I have a party with only Dutch ppl and there will be only one non-Dutch speaking person, the majority of the guests will revert to speaking English (true fact - I've seen this in nearly all the parties in which all the guest minus one were Dutch); among themselves with the non-Dutch not around, they of course will speak Dutch to eachother, but as soon as that person joins, conversation turns to English - that's imo just a matter of common courtesy. Therefore I cannot really understand why some ppl don't do this. It's not only the Poles that do this, also the Spanish, Italian and French are notorious for this. I appreciate the fact that ppl don't master the English language well, but at least an effort should be taken. But maybe that's just me.