21. The system of claim 18 , wherein the operations further comprise: identifying a plurality of first phrases in a first natural language; generating respective second phrases in a second natural language by translating the first phrases into the second natural language; identifying respective paraphrases of the first phrases by translating the second phrases back into the first natural language; and building a translation model trained on the first phrases and the paraphrase of the first phrases including using the first phrases as a source language for the translation model and the paraphrases of the first phrases as a corresponding target language for the translation model, wherein using statistical machine translation to translate the initial search query into a translated search query comprises providing the initial search query as input to the translation model trained on the first phrases and the paraphrases of the first phrases and receiving the translated search query as output from the translation model trained on the first phrases and the paraphrases of the first phrases.
18. A system comprising: one or more computers; and a computer-readable storage device having stored thereon instructions that, when executed by the one or more computers, cause the one or more computers to perform operations comprising: receiving an initial search query including one or more terms; using statistical machine translation to translate the initial search query into a translated search query, the translated search query being different from the initial search query and being in a same natural language as the initial search query; identifying a first term occurring in the initial search query that was replaced by a corresponding second term occurring in the translated search query; determining that one or more context terms that occur adjacent to the first term in the initial search query also occur adjacent to the second term in the translated search query; and in response to determining that one or more context terms occurring adjacent to the first term in the initial search query also occur adjacent to the second term in the translated search query, designating the second term as a synonym of the first term when the first term occurs adjacent to the one or more context terms in search queries, wherein a search engine expands a particular search query to include the second term in response to a determination that the first term occurs adjacent to the one or more context terms in the particular search query.