1. A translation system for translating a document comprising: a first replacing unit that replaces an original phrase having a predetermined attribute with a substitute phrase; a memory that stores a translation of the substitute phrase generated by the first replacing unit; a first translating unit that translates the original phrase; a transmitting unit that transmits a document including the original phrase substituted by the substitute phrase; a second translating unit that translates the document including the original phrase substituted by the substitute phrase; a receiving unit that receives from the second translating unit a document translated by the second translating unit; an extracting unit that extracts from the memory the translation of the substitute phrase in the document translated by the second translating unit; and a second replacing unit that replaces the translated substitute phrase with the translation of the original phrase generated by the first translating unit, wherein: the substitute phrase is chosen to maintain semantic properties of the original phrase; and the translation of the substitute phrase stored in the memory is visually different from the substitute phrase.