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.
Sapkowski uses many motifs just unknown in Poland until very recently. For example for his noted Arthurian obsession, it is known that there was another big fan in what is today Poland (due to her having Lancelot-Grail frescos in her tower) in the 14th century or so, but besides that singular instance with no cultural legacy Arthuriad only really arrived in Poland by the way of the American and British late 20th-century pop culture going global. The core of The Witcher is Arthurian Romance + Tolkien + Moorecock.
I'm not going to argue on polish history and culture because you will win. However, there are a lot of things that seem old and are new in every culture. Santa Claus in the US really didn't kick in till the 50's. Ireland didn't really celebrate St Patrick's day until the late 90's.
Claiming something is from another culture therefore not the culture doesn't work.
Arthuriana was introduced in Poland in the 1980s-1990s during the SF/fantasy boom and the fascination of the Western culture that wasn't suppressed anymore (for example during Stalinism one could be even jailed for things like being found listening to jazz and the beatniks were literally enemies of the people, while in the 1980s it was trendy among the youth to be completely openly and outwardly members of metal or punk subcultures without much consequences, with Polish and bands holding concerts legally). Just to remind you it was so completely foreign until just few years before Sapkowski began writing. (Maybe besides some knowledge of Wagnerian treatments, but Wagner slso wasn't halal for the hardcore Communists due to how much the Nazis enjoyed his stuff.)
Now, it's like to say "it's now also Japanese culture" (King Arthur is hugely popular there nowadays, not just Fate and stuff but also scholarship), making people say the actually explicitly Arthurian Witcher is thus set in "medieval Japan".
(And the Polish Witcher TV series actually did give Geralt a katana, which was often ridiculed at the time by the Polish fans, similar to how the retards at Netflix now gave a katana to their Chinese Elf.)
King Arthur is absolutely not the Polish (non-pop) culture. There were medieval Arthurian (Tristanian) works written in Czechia and Belarus (Ruthenia), but in Poland there were only some books imported from France or Germany if which none even survived. Just like "stories featuring Avalon and Camelot" is certainly not what you think if you hear the phrase "Japanese culture".
Geralt's katana sillyness: https://images.kinorium.com/movie/shot/239237/w1500_48798595.jpg
Look up Captain Kronos Vampire Hunter. It was a 1970's hammer horror film.