IIRC, Alsace-Lorraine was under Prussian/German control from 1871 until 1918, but was fairly mixed for centuries before then. I guess you probably know that already.

Yes, but there was quite a bit of resentment against that German control (Look up "Zabern affair" for an example) And after WW1 there was resentment the other way — until mustache guy killed all pro German sentiment there....