1. An operation device for, when a user selects a predetermined language key, switching display character sets of a display language displayed on a screen to translated character sets of a translation language specified by the predetermined language key, the operation device comprising: a language switch portion configured to judge whether or not all of the translated character sets of the translation language specified by the selected language key exist in correspondence with the display character sets of the display language; and a display character retrieval portion configured to check whether or not a same character set as a display character set of the display language corresponding to a missing translated character set of the translation language exists in other display character sets of the display language when the language switch portion judges that not all of the translated character sets of the translation language specified by the selected language key exist, wherein when the display character retrieval portion determines that the same character set as the display character set of the display language exists in the other display character sets of the display language, the language switch portion substitutes another translated character set of the translation language, which corresponds to the same character set that exists in the other display character sets, for the missing translated character set.