1. An image forming apparatus comprising an installer portion, the installer portion being capable of installing a translation data application and an add-on application on the image forming apparatus, the add-on application for customizing functions of the image forming apparatus, wherein: the translation data application installed by the installer portion has a data structure including: a translation data file containing multiple terms in one or more languages; and a definition file specifying an access method for the add-on application to access the translation data application and a function for the add-on application to extract a target term in a target language from the translation data file; and upon activation of the add-on application installed by the installer portion, the add-on application performs the following operations: accessing the translation data application by the access method specified by the definition file, the definition file being included in the translation data application; extracting a target term in a target language from the translation data file by the function specified by the definition file; and generating a display screen using the extracted term.