I don't think a surviving Roman empire would necessarily mean linguistic stasis though. Frankish influence would still occur, even if they are made loyal to the Roman state. Imagine if something similar Lex Burgundionum Romana with the Franks to the North, and you would have two entirely different Foederati Kingdoms within the Roman empire. While throughout the rest of Gaul you would have far more Romanized sections where Gallo-roman culture is left in tact.
Statis? No. But it wouldn't be no longer French or Occitan, it would be something else, probably even one romance language for the whole Gaul.
Foederati Kingdoms had a serious tendence to not stay foederati but rival kingdoms. (Furthermore, Franks WERE loyals to the empire OTL, but not at the point to not give a try invading a territory, critically with an WRE who would have to move troops and ressources everywhere to survive)
Then maybe a POD where Constantine III (a romano-brit emperor) some how breaks Gaul away from Rome. Still it's likely that the Huns would end up invading anyways. It could still satisfy the challenge for about 50-60 years.
And it would end as every break away with rome : or being by force retaken, or being invaded by germanic peoples.
Furthermore, there's a reason why Romano-Brittons never tried to make a break-away empire with Gaul : disproportion of forces. If Constantine III ruled Gaul, it would have not taken Romano-Brittons to defend it, but the reverse.
Of course, it would have been simpler to give up Britain for Gaul, as we're talking of exchanging Tijuana for New York regarding the possibilities.
For Brittons, I consider OTL as quite of a Briton-wank already. They had a territory that was the double of their linguistic and cultural region (even triple if you consider High Middle Ages). For more, you need more Brittons, concentrated enough to not be assimilated by the gallo-roman population, and you'll have trouble to do that.
Armorica wasn't particularly rich (not talking of the regular Saxons and Frisian raids) and the land didn't allowed much the concentration of population. More immigrants than OTL would means dispersion of groups, and likely assimilation of theses.