The only way to be certified as meeting the UN quality requirements for a translator is taking the UN examinations for translators or interpreters. Once you've pass you can apply to any UN agency or offer your service as freelance. The exam is quite tough (I've taken it twice already) and the examination is held every two or three years. If you are one of the few lucky translators selected, it's a big achievement because all UN agencies have this examination as main requirement for translators or interpreters.
Do you have any idea about where I can find info about these exams?