My understanding is that most of the romance languages were considered vulgar languages, and that scholarly men and elites spoke and wrote in Latin and Greek. For example Newton's Principia was written in Latin, which was customary at the time. By the 19th century, "pardon my French" becomes a phrase, indicating its used to put on airs and show off class.
Hi, not discouraging other responses here, but I've dug up a few older threads talking about the influence of France, which will hopefully give you a start, and perhaps a few usernames to ask followup questions of.
Side Question: Does French being considered “classy” have anything to do with it being considered the Lingua Franca of Europe before English?