Also Sapkowski himself often commented on how little of his inspirations were anything Polish or generally Slavic. It's mostly Western and Northern European.

When he began writing about other things, he wrote about King Arthur (again) and about the medieval Czechia (The Hussite Trilogy). His themes are quite extremely non-Polish for a Polish author, because most Polish authors do write about Poland (and also most Polish authors are completely unknown elsewhere for this very reason as Poland is such a globally niche and unappealing subject) while he doesn't.

How many other Polish fantasy (sci fi, alternate history) authors did you as much as hear about? Yeah, none, and that's precisely because usually they really write about Poland, and so no one outside Poland cares about it. (Maybe you heard about Lem, who didn't.)

Sapkowski's books are globally successful because they're not Polish but just European, using Arthuriana, assorted various legends and continental history, and Tolkien's modern fantasy (and Moorecock).

It's not any "medieval Poland", it's another world altogether with people and creatures from different worlds and most have Germanic and/or Celtic names.

Ciri (Greek name) in the books repeatedly visits ours world (Earth) in various places and eras, and I don't think it's Poland even there (unless the German owned Prussia counts), instead she goes to France and so on. It ends with her in King Arthur's Camelot.