Spanish will help you out the most.

 

Spanish. You're most likely going to meet people who speak Spanish instead of the other languages.

 

I am thinking of going into accounting also :D! I would take Spanish because it is spoken in many different countries by many different people.

 

China has 3x the amount of people than the US
http://listverse.com/2008/06/26/top-10-most-spoken-languages-in-the-world/
Statistics say that 1Billion+ speak mandarin while 392Million speak spanish
Mandarin is harder than spanish in my opinion (being born asian)
I would go for mandarin, if you are up for the challenge, but spanish is good if its easier for you

 

For business in America, Spanish will do you the best. Outside of America for business, there is no one good language.. but of your list, Mandarin or French would probably do the best. Not many people speak Latin (last I knew) commonly.

 

okay, before I list why you should take mandarin, let me point out firstly that; I take it, and its worth it, but its hard, don't be prepared to slack.

stanford will look highly on that language
china is a growing economic power, and constantly is growing into the US economy, whether that be as trade contracts with US businesses or simply investing in american business
china has a a ton of people, more by the second, trust me in the future, you will want it
yes its hard but worth it
nice to show off
since you want a global business, chinese is especially helpful since as I stated, china is growing like a pig on bacon steroids
also for the people that say spanish, if he wants a big global business, hes not going to speak with many high up spanish business man, more chinese high ups, then spanish... Sounds stereotypical but its true, besides telecom, theres not many big international spanish originating businesses...
btw, I got same college dream and same business dream... Rival! :) jk, best of luck