The first foreign language you want to learn is usually quite obvious, but beginning with the second there's usually a choice to be made. As a general advice, a language that will be both useful and easy to practice is advisable. If you are lucky enough to live in New York or London, practicing any language should be possible. Here's a list of major languages with some observations to help you assess if that's what you want to learn. I recommend you to read my Language learner's guide before starting, as it contains definitions of all the specific terms used in the individual language pages.
Color broadly refer to linguistic group.
The most useful of all languages, pretty easy to learn and a breeze to practice. Easy, logical and useful language that is on top of most people's list. Immensely difficult, be it in its maddening writing or most tricky pronunciation. Learning it is the language achievement par excellence . Rather difficult but sharp, accurate, great for business and giving access to a wealth of culture.