I wouldn't mind Poland merging with any of its neighbours but Russia and Germany. But I don't think any country would be interested, because apart from the mentioned two and Ukraine all our neighbours are much smaller than Poland and would be an addition to my country, not an equal part of the merger.
So Czechs are OK, at least Upper Silesia would be united this way and our languages are mutually understendable.
Slovakia is OK, they have very similar language to polish
Ukraine is OK, we have many cultural and historical ties,
but there is much mistrust and I don't think it could ever be possible.
Belarus is OK, there is considerable polish minority there, and Belarusian (not many Belarusians left speaking it) is close to polish, there are many cultural and historical ties as well.
Germany and Russia are too big.