15. A cross-lingual search ranking method, comprising: receiving a search query of a first language; retrieving a first set of documents in the first language based on the search query; retrieving a second set of documents in a second language based on the search query; 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 the document, the unified ranking function comprising a cross-lingual generative probability that is a summation calculated by multiplication of a term descriptive of a probability of translating the search query of the first language to the second language with a term descriptive of a monolingual query generative probability of the search query being in the second language given an observation of a 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.
15. A cross-lingual search ranking method, comprising: receiving a search query of a first language; retrieving a first set of documents in the first language based on the search query; retrieving a second set of documents in a second language based on the search query; 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 the document, the unified ranking function comprising a cross-lingual generative probability that is a summation calculated by multiplication of a term descriptive of a probability of translating the search query of the first language to the second language with a term descriptive of a monolingual query generative probability of the search query being in the second language given an observation of a 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. 16. The method of claim 15 , wherein the unified ranking function is expressed as: 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 the document d of the first set and the second set of documents; document d e is of the second set of documents in the second language; λ i is corresponding weight parameter; 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 ).