Does anyone know if it is considered rude in Paris to lower down the back rest of the seat in bus trips?

What kind of bus? A city bus that you're on for a few minutes? I don't think the seats recline. If it's a tourist bus, there won't be French people on it. I think it's pretty rude anywhere to intrude into the space of the person behind you.

From me as a native French person, it's like in any other country and there is the same problem in the planes too.

I think the best is to ask the person sitting behind you.

It is rude everywhere.

If you gonna make someone else there sitting space smaller you a being rude.

And i am a european if you want to know.

It's considered rude all over the world by some people ...and perfectly ok by others. And if you're on a bus in Paris, there probably won't be any French people on it, anyway.