Thank you for the answer. This will help me partially, if I understand correctly.
Belgium is a bit of a difficult country (two/three languages and all), so I can use this method to change the default language of the login site, but only the default?
We also need multi-language support, since people either need a dutch or french login page. I know from the docs that in order to support dutch, I'll need to create my own messages file, but is there a way to pass the language used by the user to the frontend screen?