France is not, and has never been, ethnically homogeneous (I'm French).

It was always a mix of different peoples - Celtic, then Romans when they invaded, then various Germanic peoples (including the Franks that gave the country it's name)... even the standardization of the French language is fairly modern. We had Occitan and Provincal and Breton spoken, it's only in the past ~200 years or so that industrialization has given a "uniform" culture.