1. A multilingual document retrieval system, comprising: a first directory storing unit that stores a first directory structure created for a first language; a second directory storing unit that stores a second directory structure created for a second language; a directory relation storing unit that stores a correspondence between a directory in the first directory structure and a directory in the second directory structure; a directory retrieval unit that decides which directory in the first directory structure has a high degree of relation with a retrieval request by the first language from a user; and a multilingual retrieval unit that decides a document having a high degree of relation with the retrieval request by the first language from the user among documents belonging to a directory in the second directory structure that corresponds to the directory decided by the directory retrieval unit.