It wasn’t that they yearned for royalist ideals to be reinstated,
It literally was, though. The language conflict you're describing didn't hit full swing until the Third Republic. The revolt in the Vendee was largely a reactionary movement by peasantry who were miffed at the idea of the bourgeois taking the place of the traditional aristocracy and clergy. The Vendee itself spoke lang d'oil and is, in fact, one of the original regions of lang d'oil.