It's amazing how quickly they get a grasp of the language when it comes to what is in it for them! You could reminder her that au-pairs are not employees, they are staying with you to be included as part of the family in order to learn the language in exchange for rent free board and lodgings and some pocket money.
If you are new to it you might like to buy a copy of Successful Au Pairs by Hill Matthews. I found this a fantastic help in answering lots of questions that cropped up that I wasnt sure how to respond to. Good for drawing up what you want them to do before they come and what you will do for them. I still use it now six au-pairs on!