How can a user permanently change the language? Currently you have to do it after every login.
You mean the interface language?
Yes, the interface language
Interface language setting is saved in a cookie. Does your web browser automatically delete them?
Yes, cookies are always delete when the browser closes. I dont know anybody who allows cookies to outlive a browser session. IMHO a cookie is the worst place when the user is logged in. Apart from cookies getting deleted, the user has to set the language on every single device - how tedious is that? Settings need to be stored in the user account.