The most spoken language in the world is Mandarin Chinese, and increasingly schools in other countries are offering Mandarin as an option. However, it hasn't yet been adopted as a second language by many countries. After Mandarin the most spoken language is English, and this is still the language most likely to be taught and used as an international language.
In some areas, notably the Americas, Spanish is fast overtaking English as the most used language. Worldwide it is the third most spoken, and as a business language it is getting more and more important, so Spanish is another good choice. Hindi and Arabic are the next most common, and both have many speakers outside their countries of origin, but as an international business language you are probably still better off with English, Spanish and, in many parts of the world, French. However, things are changing rapidly and it may be worth thinking about a course in Mandarin or Arabic in particular.