We were working on a Mendix application that supports two languages (Arabic and English), we already translated all the required text for both languages. But there’s an issue where when we attempt to login(sign-in) using an incorrect username or password the error message that appears only takes the default language selected of the application instead of using the correct translation of the selected language. Please refer to the attached picture below for more insight.
To remedy this create a page containing the authentication widgets with anonymous access, in other words create a custom login page. From there you should be able to control the language for the sign-in failures.