It happend once: a user asked to be added to the waiting
list at the same time other user was returning the item.
So the user ended up waiting for the user he was already
using.
The language were set only on session. But now it is
stored with user profile, on DB.
It is important as now I can send alert e-mails to each
user on their own languages and not the activer
user language.
Also, wherever the user logs out and logs in again,
it will see the same site locale.
