I do see your point, but I thought he meant in situations where the whole notice should be in Welsh (Cymraeg). Also, countries usually have some say in what they are called. I remember Rhodesia becoming Zimbabwe and Burma becoming Myanmar, or rather getting the world to use the name they should have had all through. So why not be Cymru???
I suspect I began before @aran and @Iestyn were twinkles in their Mam’s eyes and I entirely agree with your post!