You do realize there were large areas made up of German speaking slavs/prussians throughout Poland and Czech republic. The borders in these lands have moved and shifted for centuries... I always use the example that Mozart is considered a German composer... because Salzburg used to be part of Bavaria, not Austria.
What has that got to do with it? The Germans, Poles, Russians, etc are all Indo-Europeans that intermarried with each other for centuries. Even the hungarians are majorly Slavs genetically (with significant German and Romanian admixture)