To make it worth the effort you should know English at nearly native level (to get by) and some other language, preferrably rare. Chinese or Arabic are perfect choices. Since there are not many people around who know these languages the pay will be much higher than it will with English. Also, some kind of technical education might help enormously since the most of the stuff I translated was technical documentation and you must know how this stuff works in order to translate it correctly. The more sophisticated the original text is the higher they pay. Good (or at least basic) knowledge of Latin will also help - it is a common ground for nearly all European languages.