A display device (2,22) comprising: a display section on which an authentication screen including an ID input field is displayed; a display control section that causes the display section to display the authentication screen; a screen data storing section that, with the authentication screen being prepared for each of a plurality of display languages, stores in advance screen data of the authentication screen for each of the plurality of display languages; a user data storing section that stores in advance user data obtained by collecting, for each of a plurality of users, data that associates a display language selected by a user out of the plurality of display languages and an ID of the users; a character input section for inputting a character to the ID input field; a retrieving section that retrieves from the user data, in a state in which the authentication screen is displayed on the display section, before the input of the ID to the ID input field is completed and every time a predetermined number of characters are input to the ID input field by the character input section, the ID including the characters input to the ID input field in order from a first-input; a display language determining section that sets, as candidate IDs, the IDs obtained as a result of the retrieval and determines whether all the display languages selected by the users having the candidate IDs are the same; and a coincidence determining section that sets, when determination is made that all the display languages selected by the users having the candidate IDs are the same, the display language selected by the users having the candidate IDs as a first display language, and determines whether a display language of the authentication screen displayed on the display section coincides with the first display language, wherein when determination is not made that the display language of the authentication screen displayed on the display section coincides with the first display language, the display control section reads out, from the screen data storing section, the screen data of the authentication screen for the first display language and causes the display section to display the authentication screen for the first display language, in place of the authentication screen for the display language displayed on the display section. a display section on which an authentication screen including an ID input field is displayed; a display control section that causes the display section to display the authentication screen; a screen data storing section that, with the authentication screen being prepared for each of a plurality of display languages, stores in advance screen data of the authentication screen for each of the plurality of display languages; a user data storing section that stores in advance user data obtained by collecting, for each of a plurality of users, data that associates a display language selected by a user out of the plurality of display languages and an ID of the users; a character input section for inputting a character to the ID input field; a retrieving section that retrieves from the user data, in a state in which the authentication screen is displayed on the display section, before the input of the ID to the ID input field is completed and every time a predetermined number of characters are input to the ID input field by the character input section, the ID including the characters input to the ID input field in order from a first-input; a display language determining section that sets, as candidate IDs, the IDs obtained as a result of the retrieval and determines whether all the display languages selected by the users having the candidate IDs are the same; and a coincidence determining section that sets, when determination is made that all the display languages selected by the users having the candidate IDs are the same, the display language selected by the users having the candidate IDs as a first display language, and determines whether a display language of the authentication screen displayed on the display section coincides with the first display language, wherein