I remember reading 17th century letters from Dutch diplomats. They used French to write back home- except when they talked about money. Such matters required a higher form of communication.
There's a lot of french in tolstoy too, and I believe most of the dialog can be inferred to be in french but is in russian for literary reasons. French was the daily home language of the russian imperial aristocracy. They spoke russian with an accent.
Lingua franca actually comes from the Mediterranean Lingua Franca [1], which was mostly derived from Northern Italian languages, not French. French eventually became the "lingua franca," but it wasn't widespread (or even clearly defined) when that term started being used.