That's indeed a very odd choice to read in French. What's the point of learning French to read translations of English works? It's very long, to begin with, and while not badly written as such it's nowhere near the interest of Tolkien's original prose, yet I doubt a non-native speaker of French can really judge of that.

It's not a very "useful" book to learn either. The French of the translation is very stuffy and archaic (even native speakers have to look up words - I sure did the first time I read it at 12) while the subject matter makes it not terribly useful to read other works in the same style of prose. In the stuffy/archaic vein, there are countless much better written works originally written in French I'd recommend to practice French over the translation of LOTR. The quality of that translation doesn't come close to that of writers like Balzac, Hugo, Flaubert, Maupassant.

It's ironic a little, because LOTR is a book the French reviewers (then and now) have very often told the people who have some English to do themselves a favour and make the effort to read in English (despite the challenging vocabulary and archaic grammar in places).