A Bilingual nanny is one that speaks another language in addition to speaking English, the idea being that you employ them as a nanny to give your child exposure to the second language as well as English, thus giving them a great head start in life by making your child a natural at the second language as well. The nanny will help the child to learn the new language as they are being taken care of, and this can be in any capacity that they are working as a nanny. So whether they are just part time, or full time, live in or live out, then the exposure your child gets to the second language can be invaluable.
You can either go for a nanny from the country for which you want to your child to learn the language or you can take someone who is qualified in the second language. It is important to make sure that you have vetted the nanny and know that they are someone who is going to be good for your child, particularly if you don’t speak the second language yourself. There are nannies available who speak a wide variety of second languages so getting the language of your choice should be relatively easy. Just make sure that their English is also good, so that if your child is unable to understand something then they can have it explained in English. Be aware that your child may take some time to get used to switching between the two languages, and so you will need to be prepared for that.
You should expect to pay slightly more for a bilingual nanny as you are getting a second skill on top of the nannying skill that you are already paying for, but it will be worth it I am sure.