1. A translation system for translating a document, comprising: a dictionary management unit for managing a plurality of categorized dictionaries classified according to predetermined categories; a phrase extraction unit for extracting a noun phrase from said document; a registration category selection unit for selecting a category into which said extracted noun phrase is registered among a plurality of categories corresponding to said plurality of categorized dictionaries, respectively; a translation unit for translating said noun phrase to generate a noun phrase translation which is a translation of said noun phrase; a dictionary registration unit for registering a pair of said noun phrase and said noun phrase translation on said categorized dictionary corresponding to the category selected by said registration category selection unit; a document category selection unit for selecting the category of said document on the basis of the frequencies of use of said plurality of categorized dictionaries in translation of said document; and a registration destination selection unit selects a category into which said extracted noun phrase is registered on the basis of the category selected by said document category selection unit.
4. The translation system according to claim 1 , wherein one of said plurality of categorized dictionaries is a base dictionary in which words and phrases not classified into any one of the plurality of categories corresponding to another plurality of categorized dictionaries are registered, and said registration destination selection unit has: a category-by-category-basis appearance frequency computation unit for computing the frequency of appearance of said noun phrase with respect to each of said plurality of categories on the basis of the frequencies of appearance of said noun phrase in said plurality of documents and the categories of the documents; and a registration destination category selection unit for making a selection as to in which one of said plurality of categorized dictionaries the pair of said noun phrase and said noun phrase translation is registered, on the basis of said frequencies of appearance of the noun phrase with respect to said plurality of categories, wherein said dictionary registration unit registers the pair of said noun phrase and said noun phrase translation in said base dictionary when said registration destination category selection unit selects registration of said noun phrase in said base dictionary.