Apparently, linguists points that the family of IE languages closest to Italic languages - like Latin - may have been the Celtic (like Gaulish) ones.
Something like how Baltic and Slavic languages where close and diverged recently... or something.
True for Celts and Latins, but appears to be wrong for Baltic.
The few remanings of Gaul language and toponymy seems to show that Gaul and Latin derives from the same branch (but remains mute about Ligurian), but the baltic is definitly an older derivation.
I remember that our teachers explained us that Lituanian served to explain changes by comparison with the Sanskrit, because the marsh's isolation somewhat protected a relativly "primitive" linguistic.
For the population of Gaul, well : assuming that the total population of the province was around 8 millons in the VII-VIII and propably much populated south than north (but much in Austrasia than Neustria), that the ethnic Franks relativly settled the same area, you have regions with an significant minority or even a short majority.
The same reasoning can be used for Frisia, quite independent culturally, politically and having a great econonomical prosperity but that use a Frankish-originiated language.
Again, Luxemburg, Franconia, Mosellan Lorraine, and a great part of Old Dutch are issued from Frankish, in the areas they settled the most.
For France, it's maybe the scaterring of frankish population, and a settlement towards west (Liddle Franke, by exemple) that increased the heavyiness of the gallo-roman (it don't explain all, and maybe that gallo-roman would have imposed itself anyway)
EDIT : A good thing would be to avoid a feudalization in the Frankish Kingdom, at least not an OTL one. If you manage to butterfly Charlemagne, and to avoid to too great Frankish Kingdom, it maybe could do it.
As the feudalisation is directly issued from Carolingian Empire, avoiding it should butterfly away at least its OTL form, and have a more "national" relied organisations (i'm thinking to the duchies of Frisians, Bavarians, Aquitains, Brittany, etc.)