1. An operation apparatus comprising: a touch panel including a display unit that can display at least a first window corresponding to an initial window displaying a plurality of setting item keys on each of which a first character string and a second character string relating to each other are displayed in a first language, 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 in the first language displayed on a setting item key to a character string in a second language differing from the first language, based on information relating to the input operation applied to the setting item key and detected by the touch panel; a different language acceptance unit that displays a language switching key including the character string in the second language corresponding to the first character string in a vicinity of the first character string displayed on the 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 the 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 the second language, based on a content of the input operation detected by the touch panel, wherein changing of the language of the first character string and the second character string to the character strings in the second language performed by the character string changing unit is applied only to the setting item key, the changing instruction of which is detected by the instruction detection unit.