I would say set out your rules early, being wishy washy and liberal is going to help nobody. This is from my own experience of mucking it up! The au pair is not your friend and you need to set boundaries as s/he might well not be able to read your social cues as easily as someone from your own culture. We only did it once, she was lovely but I mucked it up by being too "nice" and she really got on our nerves through no fault of her own.
For instance:
What food in the fridge can (I'm going with "she" here for ease, sorry!) she help herself to? Ours was young and ate everything, leading to family size to visions being depleted before supper.
Do you want her sitting with you all evening? Ours had TV, computer etc in her room but constantly sat with us.
Who can she ask for decisions? Ours would ask DH in my hearing and then come and ask me, obviously not trusting his answer, this drove DH mad.
Mornings, is waiting outside the bathroom to ask you questions appropriate? How are you first thing? This drive me mad.
Is allowing a child to cycle in flip flops a good idea? All that stuff you've learned as a parent.
Put it all in writing, set the rules so you don't have to have awkward 2nd language conversations after the fact. Good luck :-)