1) it depends on how many hours the nanny works. If your nanny works 40 hours a week that is very different from a lot of families in the area who have 60-hour-per-week nannies. If you are going to be the primary person teaching your child English, will you be there enough hours per day to effectively do that? Bear in mind that young children may be asleep from 7pm-7am for years, so if you have a 9-5 nanny, you still only have 4 hours together on weekdays. Are you prepared to focus a lot of that time together on interactive language and reading, or will you need to multitask caring for siblings, making dinner, getting ready for work yourself and so on?
2) How many years do you want your nanny? If you just need to bridge the gap between maternity leave ending and your child being old enough for the toddler room at daycare, then, again language matters less. If you are looking for someone to help raise your kids for years and years, you probably need someone who is a fluent reader and writer and speller or she won’t be able to help with homework. And yes, homework counts in many classrooms starting in Kindergarten.
3) While being bilingual has lots of proven benefits, it doesn’t magically occur just because one caregiver speaks in another language to your child. If the only exposure they are getting is your nanny, they are hearing one form of the language: adult-to-child. To effectively build fluency, they would need to speak to peers in the target language, hear multiple adults speak to them, hear adults converse, listen to books and music in the target language. Are you going to buy a shelf-full of books in a language you don’t speak and send your kid to language immersion preschool? If not, the most you will get is a kid who maybe has an easier time locking in the accent in High School. It also matters a lot how well-spoken and educated the native speaker is. Imagine hiring an American nanny who dropped out of school at 14 to get a job and had been cleaning houses and babysitting ever since. She would not be well-equipped to single-handedly teach someone proper grammar and a large vocabulary in English, but people hire foreign nannies with similar profiles all the time and expect it to result in fluently bilingual children. That is hardly guaranteed.
4) Next, this is one of those things where you don’t know what your kid’s aptitude for language is, so you don’t know whether they will be impacted. Some kids might be bright, language-oriented and excellent mimics and those kids will likely learn English just fine from parents as well as a good bit of the target language from the nanny. But if your kid is predisposed to language issues (e.g., having a hearing issue, or a physical issue that interferes with speech, or is on the autism spectrum and struggles with language generally). For any of these types of kids, having a caregiver who speaks another language or only broken and thickly accented English will magnify the problem and set them back further.
5) Finally, the issue of nanny being able to communicate with parents is not insignificant! Especially if you want this person to care for your children long-term, you will need to coordinate and cooperate to accomplish things like building a nap schedule, introducing solids, potty training, scheduling playdates with peers, and managing medications and other critical details. If you aren’t confident that you can effectively relay doctor’s orders about a child’s ear infection, for example, then this is likely not the nanny for you.