In a time where traveling that "spitting distance" took lots of time and risk yes it's jarring to have them included without an in-universe explanation, just like you had exactly zero Africans and Asians in Europe at that point in time. Maybe if they had been all mages you could have seen the logic after all they are detected the first time they use chaos and mages can make doors to teletransport.
Technically - human species dropped on this planet via inter-dimensional travel like 1000 years ago.
They are not natural population in Witcher!world and they are not necessarily spread like natural populations would.
(Book suggest they are refugees from future post-apocalyptic earth destroyed by ecological cataclysm.)
I was expecting some creepy Polish folklore since I enjoyed playing Polish horror games like Darkwood. Then Witcher tossed in tired fantasy cliches like elves and wizards. They should've hired Del Toro to handle that stuff. Christ.
While there are some Slavic monsters - like strigha from book 3 is quite close to our folklore (though it should be vividly ginger haired) - overall Witcher books are much more based on Western fantasy because Sapkowski was great fan of it - there is lot of elements of Celtic and Germanic folklore, cities and cultures are much closer to medieval Germany, many monsters with slavic names are some mutated elements using Polish names of various insects and arthropods, there are basic D&D races, because muh D&D, there is Wild Hunt, names are mostly bit shfited Germano-Celtic names - Geralt, Yennefer, Triss, Cirilla.
And Sapkowski is quite oikophobic guy even denying possibility of proper Slavic fantasy because we lack proper epic mythos like Greek mythology or Arthurian legend.
So no this is aside of few elements not distincly Polish/Slavic story. Where it's Polishness and Slavicness shines, is sort of in mentality, in mix of cynism and romanticism, in vivid use of languages in dialogues. And unfortunately most great dialogues in the books - that were just asking to be played - were scrambled because we needed this extra Yennefer backstory, so short stories about Geralt were mostly castrated, and Geralt was changed from dude who love long bit pessimitistic and whiny discussions over vodka, to this utterly introvert piece of stone.
And look Cavill managed to push much much charisma in his Geralt, and his Hmmmmms are wonderful, but most of personality of book!Geralt is gone. Dunno if they thought that Cavill would not manage to do it well, or is it result of making witchers even more hated and mistrusted and isolated from society. In books there's quite a lot of prejudice against them, but over all Geralt usually can find place for himself, he have friends around who he visits regularly during his travels, for each person who is racists for witchers, there are two who believe that night of drinking with White Wolf is wonderful idea. Like in the first episode - Geralt should know Blaviken and be friend with it's mayor and his family (which makes it even harder - when he's banished from town after slaugher) - but no, let's cut it all together because our hero is so hated and misunderstood.
Bloody SJW hacks.