17. A system comprising: one or more computers programmed to perform operations comprising: receiving a term and a candidate synonym for the term, where the term is in a first language, and the candidate synonym is in a second language, and where the second language is different than the first language; generating a term group of one or more text strings and a synonym group of one or more text strings, each text string in the term group corresponding to a translation of the term into a third language, and each text string in the synonym group corresponding to a translation of the synonym into the third language, where the third language is different from the first language and the second language; and determining, from an amount of overlap between the term group of text strings and the synonym group of text strings, whether the candidate synonym is a valid synonym for the term.
17. A system comprising: one or more computers programmed to perform operations comprising: receiving a term and a candidate synonym for the term, where the term is in a first language, and the candidate synonym is in a second language, and where the second language is different than the first language; generating a term group of one or more text strings and a synonym group of one or more text strings, each text string in the term group corresponding to a translation of the term into a third language, and each text string in the synonym group corresponding to a translation of the synonym into the third language, where the third language is different from the first language and the second language; and determining, from an amount of overlap between the term group of text strings and the synonym group of text strings, whether the candidate synonym is a valid synonym for the term. 20. The system of claim 17 , where the term is from a search query, the system further programmed to perform operations comprising: expanding the search query to include the candidate synonym; and generating search results responsive to the expanded search query, the generating including, if the candidate synonym is not a valid synonym for the term, modifying a rank given to search results responsive to the candidate synonym.