i wouldn't put yourself out there too far as far as being a bilingual nurse.
i speak english, spanish, italian and am also fluent in american sign language.
my job does not pay me one penny extra for these skills.. but i often get taken away from what i'm doing to help translate for someone... most of the time i don't mind, but once people find out you're bilingual, they call you instead of the translator line because its much easier... the other night i spent 45 minutes on the med/surg floor helping a doctor do a neuro consult.. i don't even work on med/surg!
a friend of mine is an certified interpretor for ASL and makes something along the lines of $70 per hour, but she has a bachelor's degree in it and contracts herself out to facilities since someone has to be present to sign for all public events, etc. so i think the same holds for spanish.. you have to have the education, the native tongue or the immersion.
taking basic spanish is a great idea, but spanish medical terminology is a bit tricky and here in south florida there are soo many different dialects of spanish that sometimes i have trouble understanding the patients, especially those with the more central/south american dialect.
so i would say take the spanish classes, but don't offer yourself as an interpretor.