Czechoslovakia actually has cohabited by Germans Czechs Slovaks Hungarians Ukranians Polish etc for a very long time. After World War I and the sudden decline of the Austro-Hungarian monarchy, the newly found Czechoslovakia had more germanspeaking inhabitants than slovak ones. When Germany invaded there, Prague, Brno and the some other areas were still mainly german, but obviously people also spoke czech, the same goes for some parts of Poland. Unfortunately I don't know about the other countries invaded.

For the invasion of Czechoslovakia, there was a big German minority in that country at the time, and a large part of it was pro-Hitler, so many people from that minority spoke Czech and ended up serving in the German army.