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.
They are internationally successful, because of the video games.
Look at the publishing dates of the English translation. There's a large year long gap between the first two (one short stories and blood of elves) books and the rest. I think it's something like 5 years? The book franchise was dead for the English market - you don't wait 5 years for the 2nd book of a series to translate.
The translations started again after Witcher 2 was released.
Apart from that I agree with you. Sapkowski draws inspiration from many European folklore imho especially British/Arthurian/Celtic/Irish. Lady of the Lake (Nimue), Geralt's death, Wild Hunt (Unseelie Court), Tir na Lia (see Tir na nog), Muircetach (see this guy).
I'm curiously being downvoted why you're being upvoted while you say you agree with me.
Well, the book series (short stories in the Nowa Fantastyka zine at first) had become popular in Poland in first place because it wasn't actually Polish but just mainstream (international style Tolkienian) fantasy with a few instances of what we call "smaczki" and which may losely translate as easter eggs (not quite but I don't really know how to translate it).
Very Polish fantasy lit is more or less niche even in Poland, and even Sapkowski's own later Hussite series wasn't even remotely as successful despite or rather because being set in Slavic lands with Slavic characters (it's a historical fantasy), and I only know it exists because nobody talks about it.
While Witcher was always an instant hit in Poland, with multiple adaptations in various media in the first decade alone (there was even a Polish TV series, and the original video game adaptation project of an Alone in the Dark style game with set cameras) because it was a Tolkienish English fantasy (Moorecock), reworked to be explicitly Arthurian, with only some Polish-like "smaczki" for to feel just enough "swojsko" but not too much.
Without this early succeess, there would be no games.