I couldn't find much about the origin of the Breton song. It's said to be from the early 19th century. We don't know if it's Jean-François (first name) or Jean François (1st and last names). If the guy really existed or not will remain an unresolved mystery. Jean-François de Nantes is a bawdy song (lyrics here). I put it into Google translator but the result was very poor.
Note that the pronunciation [f?ã.sw?] once was the one for "François" (Francis) and "français" (French) but "français" was pronounced [f?ã.s?] in the 18th C -but probably not everywhere in France- while "François" was pronounced [f?ã.swa] at the end of the 18th C but not everywhere either. So it could be either John-Francis or John French / John the Frenchman.
Anyway, I'd suggest a look at the 1st related thread at the top of this very page.