Historical POD: Dante Alighieri writes his Inferno in Provençal, which is plausible; he'd already been writing in it perfectly, and it was Europe's language of prestige.
Provençal remains the lingua franca with its speakers in Spain, Italy, and France adapting it- as they did in real life- to various local languages/dialects.
The French adopt a universalized version of it as their language of diplomacy,
in the process infusing it with features from their native langue d'oïl, such as:
It is Italian and Iberian-friendly, as the origin story suggests, in its morphology and lexicon.
There's also some direct influence from those languages... that's up to you to find
https://docs.google.com/document/d/1HkO ... kWwEtVMBg/