Currently there is no mechanism in the compiler codebase to have multi-language strings in the compiler. Alexis Irland proposed a patch in the above thread (
https://sympa.inria.fr/sympa/arc/caml-list/2012-11/msg00152.html ) that certainly enabled some kind of parametrization, but unfortunately the patch files (which were hosted on a personal dropbox) seem lost today. (What I have at hand is a French translation of the compiler messages made by Jacques-Henri Jourdan, but without a parametrization mechanism.)
I think that this is an interesting idea and I would personally be willing to support a well-engineered patch providing the feature (the question of whether the internationalized messages should be hardcoded in a source file or use a sort of gettext-like mechanism is delicate), but I don't know the opinion of the compiler maintainers.