Yes, there are big differences between what a "Nanny" is and what an "Au Pair" is. Most people who have not personal experience with being one or hiring one don't think there is a difference.
I will copy and paste below the differences. But, unfortunately, I don't know if there are different "labour laws" for these two different positions. Common sense tells me there would be. Perhaps you can google a government website describing the au pair program and you can go from there.
The main purpose of the Au Pair concept is to help college-aged students gain second language skills by giving them an opportunity to live long-term with a European family. Thus, they get a chance to be "immersed" in the language they are learning. It's not really meant to be a "profit-making" full-time job as you would hope for in any other type of work. Thus, you get a chance to experience "full-immersion" (free room and board) in return for babysitting and light housekeeping and some pocket money to tide you over until you leave.
Families mistakenly (I feel), look to au pairs as a nanny alternative but cheaper. This puts au pairs in a difficult position where more is asked of them than what they expect to give. Families may expect professional housekeeping/cooking/babysitting skills and the maturity of someone much older; but end up with a college-student inexperienced in all of the above!
Au pairs need to come into this with their eyes "wide-open" and realize they will not make a whole lot of money since this is not the point of the "au pair" program. There's a good chance there will be miscommunication and misunderstandings due the language-barrier and different expectations regarding child discipline, housekeeping etc. etc. There are problems in this realm even without the added second-language factor.
When I was in university, my friend (from Canada) took on an au pair job in Paris (to study French). She told me there were a lot of problems and she didn't have a smashing time. She was only in her early twenties. When I was that age, I didn't have a clue about housekeeping or childcare. I personally would hesitate hiring someone so young and inexperienced to look after my children in a full-time/live-in situation, imho. Occasional evening help I would be willing but the au pair program is only full-time/live-in.
My advice is that if you're looking for an au pair, your intent should include helping out a young student with their second language learning and not just finding cheap babysitting.
If you're looking for a nanny, then hire a nanny.
Hope this information helps someone.
PS. I have had experience with 3 live-in maids/nannies and several more that lived-out. This was in Asia. It's a huge learning curve having live-in help. People who have never experienced it have NO IDEA! Thus, I would not consider full-time nanny help (live-in or live-out) unless the lady had extensive previous experience and even better--has had experience raising her own children. There's a world of difference between someone who has raised her own children and one that doesn't have children. This is not to say that woman who haven't had children can't do a good job. But, there is a world of difference still. I know some families that will only accept someone "married" as well (a possibility in Asia, but not in Europe). I see their point too but won't elaborate here as this thread is about Europe.