Not so much a rule as a daily reminder. For you and for them! You are not French! You have different food, habits, customs, humor, and a different language! It's not always easy to deal with! French food is very different from most other foods. I am American, so I am used to certain foods and customs of eating. In France a meal can last for over three hours. In the US they generally last about an hour. They have different habits, like their means of discipline as well of their over all manners. In the US most people are very success oriented. What I mean by that is that they mostly point out the good things that you do. They don't make you think that you are doing perfect work, but they do try to emphasize where you have done well. French people mostly point out where you can improve, and then forget to tell you where you have done well. This needs to be spoken about with your French family and discussed regularly. They are French, it's the way they are. But you are the au pair (and not French) so you need to remind them often that you have needs as well! As for humor, every country has different humor. The main problem is that since you aren't French you aren't going to understand the humor. How I cope is that I laugh when everyone else laughs.
Language is the big thing. It will give you a headache, literally. Having to think in a different language al the time is EXHAUSTING. Mentally, emotionally, and physically. You would be surprised how tiring it is. My advice is to bring some books in your mother tongue. Don't read them all the time, but they can help. And also sleep as much as you can. Don't stay up too late after you put the kids to bed, and don't be afraid to take a nap if you need it.
The French have a lot of different customs, but if you just speak openly with your family about what you are struggling with, things will get better.