1. A method for updating the vocabulary of a speech translation system for translating a first language into a second language including written and spoken words comprising: adding a new word in the first language to a first recognition lexicon of the first language; associating a description with the new word, wherein the description contains pronunciation and word class information; and updating the new word and the description in a first machine translation module associated with the first language, wherein the first machine translation module contains a first tagging module, a first translation model and a first language module, and is configured to translate the new word to a corresponding translated word in the second language.