In France, you just leave "something" but nothing to see with 10 or 15%, some people just don't do it, but that's true, the waiter or waitress count a bit on this extra money, i know it in part because i sometimes helped a friend in a bar.
But when my parents in law are in France (they are english), i always have to tell them there's no obligation to leave money, just a coin or two, but they always want to leave something like 10% of the bill, when sometimes that's happened, the waitress was really really please !