The voice acting was so unbelievably crass I just couldn't continue.
I know how you feel, but i managed to stick with it. Paid off in the end as it really is a very good game. I agree that the voice acting and translation is cringeworthy sometimes though.
This has been asked a lot recently, and as i have said in other threads, Like Fenris said, it is not essential to the plot and a quick read on wikipedia will suffice in terms of story.
However, i would still recommend playing The Witcher as it is a very good game well worth playing, and you will know more about the world and be better prepared for TW2