Your Au pair should be treated as a daughter of the house, taking her meals with the family and sharing their amusements. Under a proper "Au pair" arrangement the relationship between the hostess and the girl involves acceptance of social equality and is not founded on a mistress-servant basis. It follows that the girl is expected to observe the customs and way of life of the household and to respect the authority of the host and hostess.
The success of an arrangement depends on flexibility and goodwill on both sides. The Au pair has come to improve her language skills and will appreciate being talked to and her mistakes explained. Be aware that she will probably be homesick when she first arrives and will need time to settle in.