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.
This is somewhat debatable as greater Slavic lore is the basis for a large portion of European lore, but Slavic lore can also be traced back to ancient Greek and Egyptian ties as well.
Yes, it's to some degree overlapping, but then there's the matter of naming. For example, as noted in https://twitter.com/Dataracer117/status/1607649036269649920 that I just saw, the elves are traditionally Germanic creatures (and got popularized in modern fantasy by Tolkien, along with dwarves and orcs) and in the Witcher they have Irish names and Irish connections. (Nothing "Polish" about them too, just like with almost everything else.)
You’re overlapping polish over Slavic. Slavic lore is probably the most normalized in most fantasy and even Christian literature. The modern depiction of the devil is actually based on the Slavic depiction, same as when you think of witches, etc. this is mainly due to the geographic location of the Slavic region where it was the central hub to have numerous influences such as Ancient Greek, Egypt, Khazaria, Franconians, etc.
And also there's for example a Polish version of the basilisk legend (bazyliszek), and actually more than one version, but no one's going to say how basilisk is a "Polish" creature.
Actual-Polish legends are stories like Sir Twardowski (Pan Twardowski), a hard drinking noble wizard having bizarre adventures like going to the Moon with the Devil or what not (if you think of Terry Gilliam's film The Adventures of Baron Munchausen you're spot on), or the old legend of the evil king Popiel who was eaten by mice in his tower after being cursed for reasons (I don't remember). Nothing very appealing to the global audience.
It's also the case of most local European legends and entire mythologies, very few are even remotely as appealing as Greek, Arthurian, or Viking. For example even a relatively popular Irish being there only to a degree.
And a big reason is neither the Romans nor Greeks ever reached Poland, so they didn't record anything and there was nothing written before the Christianization in 966 after which still almost nothing was written about the local culture for a long time (and the Germans, having civilized themselves after cannibalizing Rome, earlier evidently didn't find it interesting too).
The Glagolitic language which is the precursor to Cyrillic was the first written language of the Slavs and developed by the Greek orthodoxy. Rome was the creator of the word sclavus and the Byzantine sklabos which means Slavic, and is the root word for every Latin based languages word for slave. Again you are putting polish before Slavic when is should be reversed, it is akin to saying Frankonian over Germanic
Polish/Slavic mythology witches (wiedźmy) traditionally were pretty much just hags in the huts in deep woods. That's how they appear in the disntinctively Polish/Slavic legends I know. Prominently, Baba Yaga (Baba Jaga).
I think sorceresses (very negative czarownice) is a foreign import, as is enchantresses (much more positive czarodziejki). Via Germany at first (along with limited witch hunts - polowania na czarownice, Hexenjagden), and the French culture as it became the culture of Warsaw (the countryside nobility was instead into an Oriental-like bizarre "Sarmatian" culture influenced by the Ottoman Empire: https://culture.pl/en/article/the-elegant-downfall-of-the-polish-sarmatians).
The enchantress/ sorceress was Ancient Greek and recycled first through Roman mythologies then later adapted by Germany