1. A method of translating a word sequence from a source language to a target language on a device having a processor and a memory comprising a source word index for the source language, a target word index for the target language, and a translation mapping, the method comprising: executing on the processor instructions configured to: store in the memory a word index comprising, for respective words of the language: the word stored at an index location in the word index, and a word mapping that identifies the index location of the word in the word index; store in the memory a translation mapping identifying, for a word index sequence comprising at least one index location, the translation of the words respectively located at the index locations of the word index; for respective words of the word sequence, identify the source index location of the word in the source word index; using the translation mapping, identify a translation of the source index locations of the words of the word sequence, the translation comprising at least one target index location in the target word index; for respective target index locations, retrieve a translated word in the target language at the target index location in the target word index; and present the translated words in the target language.
2. The method of claim 1 : the translation mapping comprising: a phrase table mapping respective word sequences in the source language to at least one candidate translation in the target language, and a translation model identifying, for respective candidate translations, a mapping probability of the candidate translation in the target language; and identifying the translation comprising: using the phrase table, mapping the words of the word sequence into at least one candidate translations; and using the translation model, selecting among the at least one candidate translation a translation in the target language.