I don't find the North/South contrast so relevant. The difference between eastern and western French may be higher.In terms of haplogroups eastern French people are rather R1b U152 while western French have more SRY2627 and R1b L21.
In the west germanic features can only be found in Flander and Normandy while in the east it can extend to the Alps. I recently saw a woman's rugby match between Paris and Grenoble. Almost 2/3 of the Grenoble team were blonde girls while Paris had just two blonde girls in the team. In fact I think that the Franks only have an influence North of Paris. The Burgudians and the german tribes employed to defend the limes in the eastern border of the Roman empire may have had a deeper impact.
Most of native French population would be of Celtic descent but:
*In southwest France you can hardly call the local people "Celtic" since Gallia Aquitania was said to be different from the rest of Gaul, probably related to the Basques.
*In Southeast France (after Marseille) people are rather Ligurians and Greek than Celts.
Even between the descendants of Celtic populations you have to make difference between eastern Celts (Alsace, Lorraine, Burgundy, Champagne, Franche comté) related to Central European Celts (La Tène, Halstatt), The Belgae (Upper Normandy, Flander, Picardy) and the British-related Celts (Brittany).