Ticket #9437: Tweak preferred language selection.

Ticket #9437: Tweak preferred language selection.

I *think* this should be the behavior (precedence-wise):

Pre-auth
--------

Prefs will most likely be unavailable or default language is empty.
Language will be set based on language drop-down on login refresh (if
language switching allowed)
Session language is used (pre-auth session will have language set if the
login screen has been reloaded)
System-wide default used (nls config)
Finally, browser detection.

Post-auth
---------

Value in user's prefs will ALWAYS control (regardless of login screen
language selection)
Language from login screen will be used (if language switching allowed)
On initial login session is new/clean; language info unavailable here
On subsequent login, this contains the session language (code will
ALWAYS stop here after user is authenticated)
System-wide default used (nls config)
Finally, browser detection.