I had them for years in the Uk and if I have learnt anything then the best way to treat them is as if they are a cousin helping you out.
As an ex-au pair (a loooong time ago, abroad) can I echo this advice, do try integrate the au pair into the family.
I was a family's first au pair. For my first 4 months I was treated like an employee (designated hours, living accomodation with a separate front door). I was but a child myself (only 18, fresh from leaving cert)
I was there for Christmas ( my first away from home:mad

Christmas passed like a (catholic) Sunday, (and I worked as normal) - no more, because I didn't understand their traditions, and they not mine. Relations became strained - about a month later we talked, and the for the balance of the year, my role changed to that of big sis/cousin/little sis (depending!) - to the point that I was happy to return to that country and keep close contact with them for years (until they emigrated to Ireland).
If you intend hiring a non-english speaking person, a non legal requirement, but one I think would be a huge bonus, would be to enroll them in an english class - or some sort of english conversation class in the area - not only a help for english learning, but gives her(?) quick access to outside contact/potential friends, so she'll settle more quickly - for me my work permit obliged the family to pay for 10 hours language lessons per week (and I am now god-mother to a fellow students child)