https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78822
--- Comment #12 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Oh, and yet another thing, if these messages is something you pass to the user, then the fact that you mix up the diagnostic text with expanded arguments precludes translation. With using std::stringstream etc. I'm afraid it is out of luck to translate it, with build_message_string you'd need to wrap the format strings you want to translate into _(...), or add a wrapper for build_message_string that would have a *gmsgid called first argument and would invoke _(gmsgid) on it before passing it over to build_message_string (and verify exgettext picks those up properly).
