I have used bits and pieces of relatively common Latin as 'the ancient tongue' but that's about it, and I make a point of putting it in a context where its meaning is clear. Latin is a little easier because it's pretty much equally alien to everyone and therefore easier to buy as some distant Battlemaster tongue.
I wouldn't use any languages still spoken today unless there was a particular word with a readily understood meaning - for instance, Chevalier or Duc. Even then I've just seen them used and not used them myself.
