1. An operation apparatus comprising: a touch panel including a display unit that can display at least a first window displaying a plurality of setting item keys on which a first character string and a second character string relating thereto are displayed, and a touch sensor that detects an input operation on each of a plurality of software keys including the plurality of setting item keys, the plurality of software keys being displayed on the display unit; an instruction detection unit that detects a changing instruction intended to change a character string displayed on a predetermined setting item key among the plurality of setting item keys to a character string in a different language, based on information relating to an input operation detected by the touch panel; a different language acceptance unit that displays a language switching key including a character string in a different language corresponding to the first character string in the vicinity of the first character string displayed on a setting item key identified by the changing instruction thus detected, such that an input operation thereon can be accepted by the touch panel, upon detection of the changing instruction by the instruction detection unit; and a character string changing unit that, upon detection of an input operation to the language switching key by the touch panel, changes the language of the first character string and the second character string to character strings in a predetermined different language, based on a content of the input operation detected by the touch panel.