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.
11. The one or more computer readable media of claim 1 , wherein the unified ranking function is expressed in terms comprising: ψ ⁢ ( q c , d ) = { ∑ i ⁢ λ i ⁢ f i ⁡ ( q c , d c ) if ⁢ ⁢ d ⁢ ⁢ is ⁢ ⁢ d c ∑ i , j ⁢ μ ij ⁢ f i ⁡ ( q e , d e ) ⁢ g j ⁡ ( q c , q e ) + ∑ i , k ⁢ π ik ⁢ f i ⁡ ( q c , d c ) ⁢ h k ⁡ ( d c , d e ) if ⁢ ⁢ d ⁢ ⁢ is ⁢ ⁢ d e , where ranking function ψ(q c ,d) is a function of the search query q c and document d of the first set and the second set of documents; document d e is one of the second set of documents in the second language; λ i is corresponding weight; q e is a translation of the search query q c in the second language; ƒ i (q e ,d e ) is a monolingual relevancy feature function used for estimating relevancy between q e as a search query in the second language and the document d e ; g j (q c ,q e ) is a feature function associated with query translation between the search query q c and the translation search query q e ; μ ij is corresponding weight parameter; d c is a translation of the document d e in the first language; ƒ i (q c ,d c ) is a monolingual relevancy feature function used for estimating relevancy between the search query q c and the translation document d c ; h k (d c ,d e ) is a feature function associated with document translation between the translation document d c and the document d e ; and h ij is corresponding weight parameter, and i is a first index that represents at least one ƒ i (q e ,d e ) and ƒ i (q c ,d c ), j is a second index that represents at least one g j (q c ,q e ), k is a third index that represents at least one h k (d c ,d e ).