1. A method performed by one or more data processing apparatus, the method comprising: receiving a search query input through a search engine user interface, wherein the search engine user interface is in a particular language; identifying a first resource that satisfies the search query; accessing click data that identifies a plurality of requests for the first resource, wherein each of the plurality of requests is initiated by a user selection of a search result identifying the first resource; for each of the plurality of requests for the first resource identified in the click data: identifying a respective search engine user interface that received the request of the user selection identifying the first resource, and identifying a search engine user interface language of the respective search engine user interface that received the request; determining a plurality of language relevance scores for the first resource, wherein each language relevance score is a measure of relevance of a respective one of the identified search engine user interface languages of the first resource, and wherein the language relevance scores are based on a number of requests associated with each search engine user interface language; selecting one or more of the identified search engine user interface languages as relevant languages of the first resource based on the language relevance scores; and adjusting a search result score of the first resource based on the particular language being one of the one or more relevant languages of the first resource.