Multilanguage with user sessions

Hello,

I have a site where i have two languages. English is the main one. In the site i have logins for my users. Everything is working fine for changing language the way i have it, but if the user logs in and the language is english then if Greek doesnt work and vice versa.

What is the best way to achieve that? So when a user login in the site on the /en to be able to change it to the other language for example /de and work. Anybody has done this before? The way i have it now is to login as english and if you want to change it to login again, but this is not good.

Thanks in advance