If you want a widely diverse fantasy series, I feel like Amazon's Lord of the Rings is a good show to do it with. There will always be people who will bitch, but I wouldn't see a problem with it. The Witcher on the other hand? It's one of the best works of art to come from Poland. It's something that I know some of their people take pride in. It reminds me of a video I saw of a little Asian girl looking up at a poster for Ghost in the Shell, and she sees a white girl playing the Major. And she looks so heartbroken.
Just do the series and Poland justice, and keep your politics in your pocket just this once. Truth be told if they go out of their way to not cast a white girl to play Ciri, my expectations for the quality of the show may go down. Because that tells me that staying true to the source material and doing the story and it's fans justice, is not as high on her list of priorities as it should be.
Can't wait for trans legolas and Whoopi Goldberg to play Gandalf. Here's an early prediction: They're going to make Aragorn black.