It isn't useless. The parent is 100% hyperbolic, at best. The fact is just that some people living in France can't speak french properly for various reasons, but it's not a generality. The french you've learn is the official one, and if you go into a restaurant, speak "academic french" and the waiter doesn't understand you, then that person isn't a french native.

The parent is basically saying "English is useless in USA, because people in some places speak broken English". Of course it's completely absurd in both cases.

The french you've learn, everybody who went to school in France learns that exact same language.