For some reason I cannot find reliable source about that. Germany's population were in majority German, but there was also Frenchs in the west, Danes in the north, Lithuanians, and Poles in the east, among other minorities that would cause no problem for Germany.
The problem of Frenchs fighting against France, and Lithuanians or Poles fighting against Russia where other Lithuanians and Poles also lived was an obvious problem. I am pretty sure Germany solved this by sending Poles and Lithuanians to the west front to fight France, and sent people from the west of the country toward the east to fight Russia, however I cannot find a source on this right now.
Nevertheless my question is, were those people (Poles and Lithuanians on the western front, Frenchs on the eastern front) treated differently than German people (such as making them more often on the front line, and by giving them more risky missions) ? I have heard this was the case, but then again, cannot find any sources.
If so, how could the German army know which soldiers were concerned, since ethnicity was not recorded in Germany, and all citizens of the German Empire were considered "German" by the government ?