Is Maths with French a useful course?

More generally, France has been a dominant force in the development of modern mathematics (and applied mathematics/physics) over the last two centuries (see https://en.wikipedia.org/wiki/List_o...e_Eiffel_Tower), and continues to provide world class mathematics education. ENS Paris (Ulm) and Ecole Polytechnique (l'X) are notable examples, although many of the Grandes Ecoles (particularly the ENS's) have a strong background in mathematics by the nature of the courses they offer, and many of the divisions of the former University of Paris have quite strong traditions in mathematics.

So in general employment terms, yes, in specific academic opportunities, very much so. Although it's worth noting, if your plan is to generally go into business in France following maths you should definitely consider doing a masters (or indeed, diplome d'ingenieur, which despite the name can specialise in mathematics, particularly at the ENS's and l'X) at a grandes ecoles. If you end up becoming a French citizen you could always try the agregation as well, and potentially get a reasonably well paid civil service job for life, teaching (which may include teaching analysis/algebra to post secondary students in the CPGE, who will be very attentive as to their minds this teaching may well hold their future in the balance). Also just getting agregation then continuing on in academia and getting a PhD (anywhere) would be a significant feather in your cap (like Part III at Cambridge, which you could also theoretically do before/after this).