Reposting my comments:

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. The 'true' protagonist 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 (in the aptly titled last book, The Lady of the Lake).

Now, 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, probably none, and that's precisely because usually they really write about Poland, and so practically no one outside Poland cares about it. (Maybe you heard about Lem, who didn't.) Sapkowski's Witcher books are internationally successful because they're not Polish but just European, using Arthuriana plus assorted various mythologies / legends and continental history (one particularly big theme is Rome vs Celts), and Tolkien's modern fantasy (and, yes, Moorecock).

Some people take it to the other extreme, like I've just read on twitter in response to an ignorant "Polish legends" a challenge of "okay, name just 2 Polish legends in The Witcher". This went unanswered (because almost no one outside Poland knows about any Polish legends), but actually I would "name just 2": strzyga & skrzaty. But both of these are not really Polish but wider Slavic, and more importantly they're just among the massively greater myth and legend body in the books that isn't Slavic at all, but from elsewhere in Europe.