I'm going to make a silly joke about a friend in a language she doesn't understand
Why do that? Are you saying anything mean about her?
Because I want to make fun of her for being a retard
You can make jokes for fun. But don't make them at the expense of others and don't hurt your friend on purpose.
I just like to make fun of people.
So you wouldn't mind being made fun of by other then? Try to exercise some empathy and put yourself in their shoes. You can make light hearted jokes without hurting your friend's feelings.