I have the utmost respect for people who are able to learn another language and utilize it fluently - holding discourse that is expressed easily and that is articulate enough to truly express one's thoughts/emotions.
I've tried to learn Spanish on my own for probably 10 years, and it just hasn't happened. I've done the Instant Immersion Software, Textbooks, Rosetta Stone, and even a few classes. Sure I can get around at an all mexican restaurant or on vacation, but I have a narrow filter in which I'm actively translating English to Spanish in my mind. It is definite gringo-tongue.
Someday as an Engineer I hope to really learn another language by being immersed in situations where I have to use it and I experience a true immersion style of learning.
I wanted to audit some of the intermediate level college courses where no english is spoken, but I simply don't have time in my schedule right now.
Kudos to those who learn a technical skill and fluently speak multiple languages!
(You can read this as, "I'm Jealous"!)
EDIT: Oh, I would pick Spanish if you plan to stay in the Western Hemisphere. Mandarin or Arabic would certainly be applicable, but they are umpteen times more difficult due to the different character sets that you will need to learn for reading/writing.