1. A method implemented by a data processing apparatus, the method comprising: obtaining a text message in a first language, the text message comprising at least one word; providing the text message to a machine translation system; obtaining a translation of the text message from the machine translation system; determining that the text message and the translation both comprise the at least one word in the first language and that the at least one word is correctly spelled; and performing one or more of the following: (a) determining a frequency with which the at least one word appears in prior text messages and, when the frequency exceeds a threshold value, adding the at least one word to a lexicon in a data store; (b) determining Bayesian probabilities for neighboring words that appear before and after the at least one word and, when the Bayesian probabilities exceed a threshold value, adding the at least one word to the lexicon in the data store; and (c) performing k-means clustering to identify a cluster of words comprising synonyms and, when the cluster comprises the at least one word, adding the at least one word to the lexicon in the data store.