In France, if you want a waiter, you call them over. When they have time, they'll get to you. They leave you alone because most French people don't want to be bothered a lot while they're talking and eating, or reading the paper (a sentiment I can get down with).

If something is screwed up with the food, they would first probably wonder why this American tourist thinks that the French chef screwed up the dish that their grandmother probably taught them how to cook. Then they'll listen to you, shrug, and take it back and tell the manager or the chef. One of them would decide what was appropriate.

If you told them you just didn't enjoy the dish and you wanted another one, they may ask you why you ordered the first dish in the first place, and why you think you deserve a free dish. If you ask for substitutions, they'd probably tell you no, because most chefs don't want to deal with that and often the way dishes are made preclude substitutions, not to mention backing up other orders. And they'd be in the right. It's kind of stunning sometimes how privileged American consumers can act.

A caveat to this is if you're in Paris; they can be really nice or really rude based on a number of factors. In a small town, French people are really friendly and helpful.