1. A method for training bilingual word alignment model via a programmed computer, comprising: training a bilingual word alignment model for a first language and a second language, using a bilingual corpus of the first and second languages; training a bilingual word alignment model for the second language and a third language, using a bilingual corpus of the second and third languages; and estimating a bilingual word alignment model for the first language and the third language, based on said bilingual word alignment model for the first and second languages and said bilingual word alignment model for the second and third languages, wherein said bilingual word alignment model for the first and second languages and said bilingual word alignment model for the second and third languages respectively comprises a word translation sub-model, a position distortion sub-model, and a word fertility sub-model; said step of estimating a bilingual word alignment model for the first language and the third language comprises: estimating a word translation sub-model for the first and third languages, based on the word translation sub-model for the first and second languages and the word translation sub-model for the second and third languages; estimating a position distortion sub-model for the first and third languages, based on the position distortion sub-model for the first and second languages and the position distortion sub-model for the second and third languages; and estimating a word fertility sub-model for the first and third languages, based on the word fertility sub-model for the first and second languages and/or the word fertility sub-model for the second and third languages, the word translation sub-model for the first and second languages and/or the word translation sub-model for the second and third languages, and said step of estimating a word translation sub-model for the first and third languages comprises: where p CE (w c |w e ) represents the translation probability from the second language word w e to the first language word w c , p EJ (w e |w j ) represents the translation probability from the third language word w j to the second language word w e , C(w j ,w c ) represents the co-occurrence count of the first language word w c and the third language word w j , p(w c |w j ) represents the translation probability from the third language word w j , to the first language word w c , collecting the co-occurrence count of the first language word w c and the third language word w j , using formula C ⁡ ( w j , w c ) = ∑ w e ⁢ ⁢ p EJ ⁡ ( w e ⁢ | ⁢ w j ) * p CE ⁡ ( w c ⁢ | ⁢ w e ) ; and calculating the translation probability from the third language word w j to the first language word w c , using formula p ⁡ ( w c ⁢ | ⁢ w j ) = C ⁡ ( w j , w c ) ∑ w c ′ ⁢ ⁢ C ⁡ ( w j , w c ′ ) .