Methods, systems, and apparatus, including computer program products, for identifying the language of a search query. In one embodiment, the language of each term of a query is determined from the query terms and the language of the user interface a user used to enter the query. In another embodiment, an automatic interface language classifier is generated from a collection of past queries each submitted by a user. In some embodiments, a score is determined for each of multiple languages, each score indicating a likelihood that the query language is the corresponding one of the multiple languages.