I'll confirm that a young child's early babble it very, very hard to understand for non-native speakers. I stopped even trying, but then I am not a nanny and I go by the theory that if my kids really want to communicate they will make the effort to switch to the right language (although if enunciated clearly enough I *will* provide the translation for whatever word they are currently mangling).
If this was a long term hire I would ask the nanny to switch to her whatever her native language is. Your child would pick up a foreign language in the best possible way (early, full immersion) and since things would have to be kept simple, at least in the beginning, there would be a better chance for the nanny to rise beyond her own expectations of what he can do as he learns more and more. You would have to invest in some baby books in that language, which depending on what it is could be easy... or not.
Since you are not looking to the long term a talk, and modeling what you do, is probably your best bet. If the nanny can't adapt, that's a different issue, and you will have to find a new one.
