In terms of distribution worldwide, French is usually the best choice after English. This is so because it's the most popular second language for people to learn after English. So no matter where you go in the world, chances are that there will be someone around who speaks either English or French.
Spanish is useless outside the Americas and Spain, and in fact the language with the greatest number of speakers in South America is Portuguese, not Spanish.
Chinese has a huge number of speakers, but they all live in China, so it's also quite useless unless you'll be spending all your time flying into and out of China.
The useful languages are those that are very widespread geographically, and English is far and away the first on that list, followed by French.
Of course, if you are sure you'll be working only in certain parts of the world, you might want to learn languages that are widely spoken in those regions. If you're certain you'll be flying in and out of Germany, you could learn German, for instance. But if you don't know or aren't sure, or if you plan to fly worldwide, learn French.