is not 100% Polish people.
There is no a country in the world, which is 100% native, Poland is about 97%, which make It one of the most (If not the most) ethnically homogenic country in Europe. Most of the rest are Ukrainians, Germans, Belorussians - in most cases these people are assimilated and know Polish much better than their "native" language. Only some of Germans are a bit different, because they live mostly in one area and German government gives them a lot of money on keeping their language and culture. There is few Lithuanians and they live mostly in one town near Polish-Lithuanian border, I don't exactly know how that happened, but probably someone forgot to send them on the other side of the border after WW2. Generally there are no problems. Now many foreigners are coming to Poland. The biggest "exotic" minority are Vietnamise - about 50 thousand of them live in Poland.