Those borders changed hands so much. If the French side is from the eastern French region of Alsace Loraine and your other side is Luxembourg I would call that side 100% German as that side is most likely descended from Germanic tribes.
Would be hard to differentiate genetically since Central Europe is such a crossroads. Plus, there’s been a lot of migration within Germany and the historic German speaking lands, that it would be difficult to accurately pinpoint a “German” marker. I’ve always thought that “German” was more of a culture with a shared language than an genetically predisposed ethnicity unless you go way back to the ancient tribes a thousand years back or more.