The Franks were Germanic, but they were a smaller force conquering a larger, existing country. As a result, while they had some influence on the language and culture, the land of Gaul eventually absorbed and overcame them. Much like the Normans in England. So, the Latin of Gaul, which was already different due to Celtic influences, became French. Spanish, which was not Celtic influenced, remained more like Latin. Which is why Spanish and Italian are closer today than French.
Excellent. Well said.