I haven't replied yet to this thread, but i think, like many others, that there won't be so many changes in the french (which was only one of the languages spoken in "France" at that time). The only question is knowing WHO will speak it. As Paris is the capital of France, it seems quite obvious that it will be its language who will eventually be predominant.
Perhaps you can imagine a TL where as it is larger, France isn't so centralized and therefore French won't be used everywhere (like Spain with Basque and Catalan and Galician) and that the situation could stabilize otherwise.
Anyway, the real strong decline of regional languages is after the Revolution, when regional languages were considered as reactionnary, backward. This led to its progressive systematical destruction by the power (but only a century later with Jules Ferry).
You can't modify the structure of the language spoken with your POD.
Either you have to go much later (making France to recognize and promote regional language with a Revolution POD), or much earlier (more or less at the epoch of Franks)
As said by many linguists and proven since many years, the myth of "francien", the dialect of Paris which eventually became THE french, is wrong and outdated. No matter where the capital is, Paris, Boulogne or Zanzibar; the standard french was elaborated from the french (and other gallo-roman languages) dialects with a major importance for the economical and cultural centers. OTL, it was mainly the Norman-Picard and the Tourangeau; ITTL it could be with a more important part of Lorrain and Wallon, with main influence in the vocabulary (economical critically) from Rheinlander and Flemish.
For the decline of non-french languages and french dialects, its far more recents, final XIX and beggining of XX (traditionally with the WW1 at the end). Of course, among the elites, classical french was used since XV in all the territory (making the standard french quite stable at the XVII), but the popular languages and dialects still existed, with a certain vitality until the end of XIX.