Oh it's there in The Witcher, but Sapkowski changed a lot. Koshchey ('the Undying') changed from a kind of immortal lich into a kind of ueber-crab. Nekkers - in the legends of the Low Countries a water spirit that might vary (depending on place and time) from benevolent creatures to water demons - turned into, well, vaguely goblinoid forest-dwelling mystic arrow fodder.
Still, there's quite a bit of creatures never seen or mentioned in stock fantasy before. What's even more important is the 'tone' of the Witcher games.
Regarding Tolkien, problem is that people keep copying the 'furniture' of his work (often by way of D&D), but few apply his methods or, like him and other famous fantasy authors, turn to the original source materials and create something new.
When was the last time we saw a game with a story and setting like, say, Poul Anderson's The Broken Sword ? How many RPG's do we have that takes place into something that rolls, say, with a well-developed setting based on Dark Age history and Dark Age Germanic and Carolingian legends ?
Even Arthurian stuff is terribly under-utilized.
I forgot about the nekkers, killed so many of those things I stopped noticing them I guess

Koschey... Wait, I think I know that one, is that the legend of the undying guy who kept his heart in a box in a turtle in a duck or something? Still, I file that under the odd notables. I just feel like people are seriously stretching it when they talk about a fantasy world "inspired by slavic mythology". It's mostly a typical western fantasy world creature-wise with bits of slavic mythology slipped in. I was glad they did that much though.
The tone is the one thing I appreciated most about the Witcher's worldbuilding, the thing that really makes it stand out.
That's put better than I did. And you're right, they tend to borrow the 'furniture' and just try to rehash it in an interesting way (If they're willing to put the effort in). At this point its a thoughtless decision for developers. I toy with building fantasy worlds as a hobby, and it wasn't until several years ago that I realized G.R.R.M. was right "the elves have been done to death". I never even think of including them anymore. The trouble comes in resisting the urge to employ the same tropes that made me a fan of medieval fantasy to begin with. It's hard, but rewarding to try and put something coherent together that isn't "standard fantasy".
It's Polish/Salvic through and through, influenced by national history and politics, both the books and the games. Everything from war time, rulers like Radovid and Foltest, the interaction of the north with Nilfgard, stories in the quests, etc. Western audiences miss out on a ton of stuff really. When you see examples like Lady of the Lake, it's done in the spirit of Sapkowski who loved putting his own twist on well know fairy tales and folklore.
Polish/Slavic history, sure, you're absolutely right. I was talking specifically about the oft touted "slavic mythology" line.