Well, do not check the box on AMCAS, where you are indicating fluency. But I would mention it in one of your experiences, if its relevant.
Where would you mention it on the app? As a hobby?
Seriously - there is no need to hustle and learn Spanish just to put it on the application. Most schools that do look for Spanish ability want
meaningful involvement with the language and the culture, not just a quick, 4 month long CD self-learning course.
Before you jump down my throat, I'm speaking from personal experience. I can speak Spanish fairly well (not really fluent/native, though). I did a 2 month internship in Spain, had to interview total strangers in Spanish, was a language-exchange partner with an Ecuadorian woman, and minored in Spanish culture/history. Despite all this on my application, this didn't help me AT ALL in the interview process. The schools that looked at me were schools that I was confident I could get an interview at anyway. The only exception was UT-SA, but if you're a Texan then they'll look closely at you anyway.
Even if you want to learn Spanish before starting med school, if you don't have a really strong foundation, you'll forget most of it by the 3rd or 4th month. After 2 years in med school, I sometimes wonder if I'm forgetting how to speak English (which is my native language)!!
Even after my experience speaking Spanish, I can still get lost sometimes seeing patients. A lot of them speak a Mexican slang that I've never heard before, which makes me wish that I'd done the internship in Mexico. (Sigh.)
If you want to learn Spanish just for fun, learn it at your own pace and for your own enjoyment.
