I was astounded (speaking as a US kid here), to learn that French people born and raised in France didn't natively speak French, but instead learned their regional language.
Here is an example, from https://archive.org/details/malevilmerl00merl/page/150/mode/... :
> And besides, Thomas was already quite isolated enough as it was: by his youth, by his city origins, by his cast of thought, by his character, and by his ignorance of our patois. I had to ask La Menou and Peyssou not to overdo the use of their first language — since neither of them had learned much French till they went to school — because at mealtimes, if they began a conversation in patois, then everyone else, little by little, would begin to drop into patois too, and after a while Thomas was made to feel a stranger in our life.
Two minutes ago I learned that "patois" has a distinct meaning in France: "patois refers to any sociolect associated with uneducated rural classes, in contrast with the dominant prestige language (Standard French)" https://en.wikipedia.org/wiki/Patois
I am very ill-informed on the history of the topic, including the national language policies of France and Italy. I do know that Sardinian is not a dialect of Italian, but my knowledge isn't much deeper than that. ;)