https://bugzilla.redhat.com/show_bug.cgi?id=1015415
--- Comment #1 from Michel Alexandre Salim michel+fdr@sylvestre.me --- Apologies for the wrong initial assignment, I've since worked out together with the MATE maintainers that the problem only occured when MATE is launched from within gdm -- starting from startx, it picked up the LANG setting set there.
There seems to be no way to override the GDM language setting, and GDM itself was displayed in English which is odd.