What language should I learn?

This would depend on many things. Where do you live or want to live? English is a very flexible language in that it is the most widely spoken language when both first and second language speakers are included. But other languages such as Spanish, Arabic and French are also widely spoken. Of course though, what language you learn should be dependent on more than simply how many people speak it. Think about your own personal interests and what you want to do with the language - for example, do you have a business or academic interest in a particular country or region?