I probably misunderstood some aspect of your argument. Are you
envisaging a world in which gettext() is the only C-side translation
API, and all error message translations come from gettext catalogs? In
that case your suggestion would indeed make a lot of sense.

I'm coming from the expectation that the APIs from which we can get
error message translations are as many and varied as the error messages
themselves. In this world, the code that gets the error from C is best
equipped to figure out where to get its translations (if any). Maybe it
will use gettext, maybe it has to use something else.