If you add historical conflict to it then it can get much worse. "Warschau" in a non-German text would make many Polish people angry because of memories of forced Germanization.
I see no reason at all to use the French name for a Flemish city when writing in English. When writing in French, sure.
Most local inhabitants would have called it Leuven, as it is a Flemish city. The rich people at that time, both inside and outside the city, heavily promoted French, and pushed only the French names in politics and international communication.
Even today, this is a very touchy subject.
Flanders is Dutch-speaking.
Wallonia is primarily French-speaking, except for a small part of it which is German-speaking (
The Brussels Capital Region is officially bi-lingual Dutch/French, but in practice it's almost completely French. In restaurants, shops, ... you can be sure to find people who can help you in French, but almost never in Dutch. Police, hospitals, ... are supposed to be bi-lingual, but good luck getting good help if you speak Dutch but not French.
In most places, you'll only encounter one language: your own native language. Only Brussels is supposed to be bi-lingual.
Many cities do have two or three names, but that's not because those two or three languages are actually spoken in that city.
Which places in Belgium have an English Wikipedia page with an exonym as the page title? I can find Brussels, Bruges, Ghent, Ostend. Are there others?
I wouldn't be very surprised if a couple of those went out of fashion in English over the coming decades, just like "Brunswick", "Francfort" and "Aix-la-Chapelle" went out of fashion. And "Basle" is on the way out, isn't it?
There's a lot of exonyms in Polish in everyday use.