I think the problem is the awareness of a whole world like we do or more precisely Sapkowski didn't want to give them that awareness or give it any attention, because it isn't relevant in a low fantasy. Geralt isn't saving the world, so the world doesn't have a name
They know there are or can be other worlds, but they don't know those worlds and it's hundreds of years ago, so how do they should call them? Can you say with certainty where your grandgrandgrandgrandgrandparents are from? You probably can't for sure and so can't they, but this isn't even important for them, since their known world
is their world or better their
home, e.g. Northern Kingsdoms, Nilfgaard Empire or Zerrikania. Unlike than us, who are aware of a whole planet and other planets(!), they are probably not really aware of something like that, because it doesn't concern them (at least most of them).
Of course they know there is or could something in the east after the desert Korath or even a whole new continent in the far west, but even though they know a country called Zerrikania, they don't even know much about it. We don't even know the real extent of the Nilfgaard Empire and if there is something south of it!
And that is the next problem, how do call a world or just that one continent, if you don't even know it completely? And who has the
right to do so? An empire? A religion?
When did we began to call our world in term of a planet and not in "our land"
Earth? Our world changed due discoveries like "the new world" = America, yet the name always stayed, Earth or Terra. Why? Because that's how we call(ed) our land, our ground. If you think about it, Water or Aqua is a much better name for own planet, because it is covered two-thirds in water (even though there are planets who are covered completely with water), but we didn't know it back then and it also didn't matter, because that wasn't "our world".
If someone asks me "how do you call this world?", i would say "come again? Our world? You mean our planet or my home?", because i distinguish between "my world" = "my home" and "my world" = "our planet", but that's probably the problem in the Witcher universe, because why would someone give this world a specific name, if there is only this known world for him? This is home, his world for him, there is nothing else to compare it to except for other countries.
Someone living in Winterfell won't say, "I'm from Westeros.", he will say, "I'm from Winterfell" or "the North", but if he went to Essos and someone will ask him there. He will probably say first "I'm from Westeros", given the circumstances. The term of world changed for him. But how do you call those continents together? Well, you can't, because there is no need for it.
When did people start saying Europe? When they discovered there is much more land in the south and east? So they can distinguish "our world" from "theirs"?
But to come back to the Witcher....i don't think they have the awareness of the world as we have and therefore they don't have a name for it. In the simpliest case they just call it Earth or Old/New Home. Boring, i know, but what do you expect, if you ask someone in what world he is living in