20. The one or more computer readable media of claim 18 , wherein the term descriptive of the monolingual query generative probability between the search query (q c ) and the document (d c ) is expressed in terms comprising: ∑ i ⁢ λ i ⁢ f i ⁡ ( q c , d c ) , where ƒ i (q c ,d c ) is a monolingual relevancy feature function used for estimating relevancy between the search query q c and the document d c , and λ i is corresponding weight parameter, and i is an index that represents at least one ƒ i (q c ,d c ).
18. One or more computer readable media having stored thereupon a plurality of instructions that, when executed by a processor, causes the processor to perform the following actions: for each document of a first set and a second set of documents, computing a value of a unified ranking function that is a function of a search query and a corresponding document, the unified ranking function comprising a cross-lingual generative probability that is a summation calculated based on a first term descriptive of a probability of translating an observed document from the second set of documents that is in the second language to a first language and a second term descriptive of a monolingual query generative probability of the search query being in the first language given the observed document that is in the second language from the second set of documents; and ranking the first set of documents and the second set of documents together based on the values of the unified ranking function.