I'd like some context on the linguistic shifts throughout the history of the French region.

As I understand it, The language spoken in France was originally Celtic (Gaul I think), then Germanic (Frankish Kingdom), and finally down to Romantic (France).

Was it a cultural migration/ expulsion? Just a linguistic shift? What caused it?

My best guess right now is that it has something to do with the Romans, the Holy Empire thereof, and maybe the spread of Christianity.