My friend's twins, have been since they were infants, taken care of by a Spanish speaking Nanny. They are fine, healthy, well rounded, AND are now at their ages bi-lingual. Which being bi-lingual is a wonderful gift to have.
Their Nanny, took them to toddler classes, and play dates too. All approved of course, by their Parents. These kids are BRIGHT and NORMAL kids. AND, they are now knowledgeable about both cultures. Which the parents have no problem with.
I am assuming, YOU are bi-lingual? Or have/can speak to the Nanny in Spanish? Since you said the Nanny only speaks Spanish? Otherwise, you may have a hard time communicating, if you do not speak her language.
You could, in the beginning if able, take a few days off of work, or your Husband, to "supervise" or just be a fly on the wall, to see how the Nanny handles herself, and how she and your baby get along. AND, I would think that you are fully aware on HER child-rearing philosophies and approaches... AND that you are both on the same page about it? These are key issues.
AND, you as a Mom, have to organize their activities as well... meaning, telling her where you want her to take your son, or sign him up for classes/toddler groups/Gymboree/art classes etc., so that the Nanny can THEN have a "plan" and daily "routine" with your son. That is your "job" to lay-out parameters and activities for your son/the Nanny, for example.
I am also assuming you did a background check on the Nanny? In addition to checking out her references and resume?
All the best,
Susan