7. A system comprising: one or more computers operable to perform operations including: providing a received input segment of text in a source language to be translated into a particular target language as a query to a search engine, the search engine searching an index of one or more collections of documents; receiving one or more candidate segments in the source language in response to the query; determining a similarity of each candidate segment to the received input segment; for one or more candidate segments in the source language having a determined similarity that exceeds a threshold similarity, providing a translated target segment corresponding to the respective candidate segment as a target language translation of the input segment; and for a candidate segment in the source language having a determined similarity that does not exceed the threshold similarity: identifying one or more subsegments of the input segment, providing each of the one or more subsegments of the input segment as a respective query to the search engine, and for each provided subsegment: receiving one or more candidate subsegments in the source language in response to the respective query, and determining a similarity of each candidate subsegment to the provided subsegment; using the similarity of the one or more candidate subsegments to determine one or more non-matching subsegments of the one or more subsegments; replacing the one or more non-matching subsegments with respective substitute subsegments in the source language to form a substitute candidate segment; and in response to determining that the substitute candidate segment has a similarity to the input segment exceeding the threshold similarity, providing a translated target segment in the target language corresponding to the substitute candidate segment in the source language as a target language translation of the input segment.
8. The system of claim 7 , the system further operable to perform operations comprising: for candidate subsegments having a similarity that does not exceed the threshold similarity, using a model-based statistical machine translation to identify target subsegments corresponding to the input subsegments.