This is interesting I have a similar situation. My boyfriends family is rich (well compared to what I'm used to), snobby, judgmental, well mannered, etc... My family is.....from the ghetto basically! I was raised by my grandfather though, and for that reason I'm much more well behaved than the rest of them. I was afraid of my family meeting my boyfriends family!! I know that my mother speaks her mind and doesn't care if someone doesn't like her. It's a crazy world to mix, let me tell you that.
You have to consider what it's like for your wife (or girlfriend), your family is probably a bit odd to her too. The only thing you can really do, is ask her family to tone down the swearing a bit. Tell them that you don't mind, but it might really offend your parents. I'm sure that they respect you enough to try to tone things down!
If they don't....oh well! I think that your family should loosen up a bit and have a laugh later.