In today's age of globalization, proficiency in a foreign language is certain to give you a competitive advantage and broaden the horizon of your employment prospects. The knowledge of a foreign language makes you a confident traveler and negotiator with the ability to communicate thoughts and ideas in the right ethos for bridging the cultural divide.
Almost any language would be the answer, but let us look at it statistically.
English. Of course. It is the most widely spoken language of the world today.
Chinese. Mainland china is an emerging superpower and the knowledge of widely spoken Chinese language like the mandarin would be a definite asset for doing business with our mighty neighbor.
Spanish. The second most widely spoken language in the world has the obvious potential; besides it is the language spoken in the southern and Central American countries with whom the Government of India is planning to actively pursue business with.