8. A computer program product, encoded on a non-transitory computer readable medium, operable to cause data processing apparatus to perform operations comprising: receiving a first search query comprising at least one original term in a first language; for each of one or more different languages, each different language being associated with a different collection of documents, (i) translating one or more of the original terms of the first search query into one or more translated terms in the respective language and (ii) determining, for each translated term, a score that is a measure of how common the translated term is in the collection of documents associated with the respective language; in response to determining that at least one of the one or more determined scores for the translated terms into a second language is: (i) greater than a first threshold: and (ii) less than a second threshold selecting the second language from the one or more different languages; generating a second query, the second query being a translation of the first search query into the second language; generating a combined search query using: (i) the first search query comprising at least one original term in the first language, and (ii) the second query in the second language; and obtaining a final group of search results responsive to the combined search query, wherein the final group of search results comprise search results satisfying the first search query and search results satisfying the second query, wherein each of the search results in the final group of search results is assigned a score based in part on a quality of results statistics for the second query.
8. A computer program product, encoded on a non-transitory computer readable medium, operable to cause data processing apparatus to perform operations comprising: receiving a first search query comprising at least one original term in a first language; for each of one or more different languages, each different language being associated with a different collection of documents, (i) translating one or more of the original terms of the first search query into one or more translated terms in the respective language and (ii) determining, for each translated term, a score that is a measure of how common the translated term is in the collection of documents associated with the respective language; in response to determining that at least one of the one or more determined scores for the translated terms into a second language is: (i) greater than a first threshold: and (ii) less than a second threshold selecting the second language from the one or more different languages; generating a second query, the second query being a translation of the first search query into the second language; generating a combined search query using: (i) the first search query comprising at least one original term in the first language, and (ii) the second query in the second language; and obtaining a final group of search results responsive to the combined search query, wherein the final group of search results comprise search results satisfying the first search query and search results satisfying the second query, wherein each of the search results in the final group of search results is assigned a score based in part on a quality of results statistics for the second query. 9. The computer program product of claim 8 , wherein the plurality of different languages have an order and each language is considered in accordance with the order.