In addition to Poland having roughly a fifth of it's population die in the war. It lost extensive territories in the east, that included Ukrainian, Belorussian and Lithuanian populations. To compensate for this Poland received former German land in Silesia and Pomerania, and these areas were resettled with these eastern Poles. The other ethnic groups weren't resettled. To this day you can see the effects of this when looking at a map of Polish dialects. The territories that were Polish before ww2 have consistent dialects, while in the newer territories it's just one group labeled "New Dialects".