How to show the language selector in the login page?

Hello,

I've seen that in your network: https://auth.ilesansfil.org/login
you show a language selector on the right. Since we're translating the interface to italian language (we'll submit you the files once ready) we would like users to be able to select Italian or English. How can I do it?

This is our login page: http://wifi.ptlug.org/login
but as you can see, the language selector is not available.

Thanks!