It's a controversy over the protagonist in each series, namely Geralt of Rivia and Elric of Melnibone'. Both are more than strikingly similar: both are albino, used drugs to supplement their physical/magical strength, were sorcerer/swordsmen, and, most damning, Elric was known as the White Wolf when the series was published back in the last sixties/seventies.

From what I've read of Sapkowski's works, though, to his credit, the stories and settings are completely different, although both were/are considered antiheroes. It might be that his series has had limited exposure outside of it's native Poland, or that M. Moorcock has little interest in pursuing a lawsuit (he's called the Witcher a "passing off" of his character, but that's about it).
I was aware they were published in "other languages" and suspected english was one of them. I'd like to read the novels, certainly.