First some clarification. The Habsburg Southern Netherlands' was not today's Belgium. It was roughly the area of today's Flanders: Everything north of Brussels and east of Bruges. The area west of Bruges went back and forth regularly between Austria and France. To the South of Brussels, you had the prinsbishoprick of Liege and Greater Luxemburg (Today's state of Luxemburg and the Belgian province of Belgian Luxembourg that is of roughly the same size). Both Liege and Luxemburg were independent states of the Holy Roman empire, not associated with Austria. So France would only get the Dutch speaking part of Belgium. Unless this is it risks taking on the HRE over a couple of states just because those states happen to speak a kind of French dialect.
Another point is that until WWI, Flanders didn't speak 'Dutch'. Rather every city or region spoke its local dialect which were basically Dutch in nature but were as different from the uniform Dutch of the Netherlands as one dialect from the other two towns over. This came out of history as the Netherlands needed one common language to hold their state together whereas Flanders for most of its time had been under 'foreign' administration and took on the language of the rulers for administrative and commercial purposes while keeping the own dialect for business amongst themselves.
Third thought: As I already remarked, France by time controlled various parts of the western Flemish border regions. It even has its own province of 'Flandres', which is roughly the strip between Dunkirk and Lille. This region has been part of France since the middle ages. Yet the population still holds on to their 'Vlaamsk' dialect. Today, all administrative duties are handled in French and so are most of the billboards and publications. Yet 'Vlaamsk' is officially taught in school under 'local history' just as Breton or Basque in other parts of the country. So yes, I believe if France were to control all of Flanders, this would be their model.
It has been suggested that France will push heavily for a uniform language and try to eradicate local tongues, but revolutions notwithstanding, by the time they would get serious, we would be well into the 19th century and well into romanticism. And with that the rich history of medieval Flanders would make a comeback and so would the language. It would not be standard Dutch of course but a mixing of local dialects with some French borrowed words thrown in. It will however still be Flemish, ...or 'Vlaamsk'