I've only read a few of the novels (the first two, in order of release, not chronology, and Sword of Destiny...which came out waaay late in English). I agree that the characters are significantly different in the novels. Overall, more gritty and vulnerable. I also agree that the English translations are definitely missing the verbal impact that I know has to be there in the original Polish.
For example, I've actually read 2 different English versions of The Blood of Elves. In the first, Ciri is referred to as Geralt's "Surprise Child"...which is a terribly awkward phrase in English and a clunky, linear way of referring to the recruits that witchers receive when they claim the "Law of Surprise". In the second translation, she is called his "Unexpected Child" (like the games), which is a far more elegant and (I think) correctly layered double-entendre. It's a cold confirmation that the Law of Surprise resulted in a human child (e.g. instead of a horse or some supplies), and it also draws an ironic parallel to witchers being sterile (read: "unexpected child" = "got pregnant by accident". Very dark humor, especially considering what awaits the children at Kaer Morhen. I imagine this is what Sapkowski was actually going for.)
There are other parts of the books that I sense are still missing the verbal mark. Unfortunately, there's not too much more that can be done; sometimes, idiomatic speech just doesn't translate perfectly between languages. But the stories are well worth reading! It is a brilliant fantasy world with amazing characters and moments.