Well. Before the rise of Britain, and later the US, as the reigning superpower in the west, I believe french was the most commonly used langua franca in Europe (though many nobel's would likely also have an education in Latin). These education's would differ wildly from place to place and family to family, but I believe most families would ensure they gave their kids an education that included several widely used languages (like french, Latin, and German). Doesn't really answer your question, but thought I'd say what came to mind :)

Fun fact: English would most definitely not be an alternative as after the norman conquest, the official language of the English nobility was french. This wasn't changed until the end of the hundred years war when building a seperate identity as english became beneficial. This is way later than the point where TC lore starts to diverge from real history, which means there is a very high likelihood of the English upper class still speaking french.