Is it possible to change the language of the default login page or do you have to build a custom login app for that?
Unfortunately, it is not possible to change the language for the default login page, so you will have to create a custom login app.
You can read more about custom login apps here: https://docs.appfarm.io/appcademy/appfarm-professional/login-and-configuration/login-and-authentication#custom-login-app