1. One or more computer readable media having stored thereupon a plurality of instructions that, when executed by a processor, cause the processor to perform the following actions: retrieving a first set of documents in a first language and a second set of documents in a second language via a cross-lingual search query that is based on a search query written in the first language; for each document of the first set and the second set of documents, computing a value of a unified ranking function that is a function of the search query and a corresponding document, the unified ranking function comprising a term descriptive of a cross-lingual query generative probability between the search query and the corresponding document, the cross-lingual query generative probability being a summation calculated based on a term descriptive of a probability of translating the search query from the first language to the second language and a term descriptive of a monolingual query generative probability of the search query being in the second language given an observation of a document in the second language; and ranking the first set of documents and the second set of documents together based on the values of the unified ranking function.