5. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: obtaining, at one or more computers, a pair of terms in a first language, the pair of terms being commonly co-occurring non-synonyms in a corpus of documents, the corpus of documents being in the first language; determining a set of variations for each term in the pair of terms; generating a set of known related input pairs based on the sets of variations for each term in the pair of terms; for each input pair of terms in the set of known related input pairs, translating, by an automatic translation system, each term in the pair of terms into a plurality of languages to generate a set of translated terms; adding, at the one or more computers, the set of translated terms to a blacklist of known non-synonym pairs for at least one of the plurality of languages; and determining, based on the blacklist of known non-synonym pairs, whether a pair of candidate terms in at least one of the plurality of languages are synonyms.
6. The system of claim 5 , wherein determining variations for each term in the pair of terms comprises determining one or more normalized versions of each term in the pair of terms.