This is from an interview with several cast members:
Lauren Tom: I usually throw the Chinese swearing in there to crack everyone up. I taught Billy (West) how to do it and he's perfect at it.
The Cantonese viewers out there actually know what I'm saying, it's not made up. So I make sure it's stuff like, "I'm going to beat him to death!"
But I only know certain phrases, because I'm not fluent. I only know the things that were said to me when I was little. Lovely things like, "I'm going to chop your head off!"
Maurice LaMarche: So nothing sexual, or to do with excrement, then. Just horrific violence. We've all seen that on TV.