Your whole thesis hinges on Western culture being a “thing” and yet, you nullify it right away by bringing up a maximalist argument of land re-acquisition.
Let me take this even further then: If borders are deliberate, and cultural boundaries are artificial, then why not make the Witcher more diverse? After all, the Polish didn’t claim sole ownership over their land throughout the totality of human history.
See where this goes wrong? The point here, is that the Witcher draws on modern conceptions of medieval fantasy (one could argue heavily inspired by Tolkien’s mythos) and the novels themselves criticize that very romanticism. So it would only make sense to me that an adaptation would include people of color as a way of further pushing the envelope on its cultural critique of cliches of medieval fantasy — that other cultures, and people with browner skin lived through it too. It doesn’t have to be either Aladdin or Lord of the Rings — there’s room in-between for so much, and given the Witcher’s existing subtext about racial discord after the Convergence of Spheres, it only makes sense to represent brown people more than dragons, which by the way, have a non-European provenance.