The family's typical menu may well be alien to the Au Pairs' normal diet. Try cooking dishes more familiar to the Au Pair and then gradually introduce your family’s diet. Why not let the Au Pair cook their own favourite dishes?
Every family and every au pair is different, so it is impossible to provide a definitive list of every concerns and grievances. Fortunately most disagreements can be resolved. Here is a list of commons issues and how to approach a way to resolve them.