It depends on what your goals are regarding your career. Mandarin is the most spoken language in the world, with English and Spanish ranking second and third, but it really depends on in which area of the world you plan to pursue your career.
You should start learning ARABIC. Practically every product has its leaflet or literature printed in Arabic. China is also one country where we are doing a brisk business. But china is one nation. Where as more then 50 Arabic speaking countries we are doing business with, So it is important to learn ARABIC rather then other languages.