5. The apparatus according to claim 4 , wherein: the second acquisition unit is operable to obtain, as the search results, a first value indicating a number of pieces of document data that include the plurality of constituent words in the first document data group and a second value indicating a number of pieces of document data that include the plurality of constituent words in the second document data group, and the generation unit is operable to generate the index on the basis of a result of comparing the first value with the second value.
6. The apparatus according to claim 5 , wherein: when a ratio of the first value to the second value is equal to or more than a first threshold value, the generation unit is operable to generate the index, which indicates that a level of adequacy of the compound word obtained by the first acquisition unit as a translated word in the field corresponding to the first document data group is a first level, and when the ratio is smaller than a second threshold value that is equal to or less than the first threshold value, the generation unit is operable to generate the index, which indicates that the level of the adequacy of the compound word as a translated word in the field corresponding to the first document data group is a second level that is lower than the first level.