An user without root privs can't set language of the user interface, and also
can't set the locale.
This is very important in multiuser workstations, where you have lot's of people
wanting to use some other than the default langauge of the box.
You can create /etc/.i18n and set your locale there.
But still it should be changeable using the graphical ui.
It has to be set at login time, and can be picked from GDM.