add Yii::t() tags to all lose strings
add yiic message configuration file and messages directory
invalidate the main menu cache when language changes
add ability to change session language from the menu
add a setting for defining the global application language and use it
when a user hasn't specified a language himself.

This way the login screen will also be translated.
show a notification to administrators that changing their own language
won't affect other users
add a language column to the user table
implement the "set as default" checkbox when a user changes language
remove runtime caching of the setting definitions. They are cached
early on before the application language has changed which makes the
Settings page always appear in English.
show a notification if the user's language settings overrides the
application language when saving settings (to not confuse the user
when the language is not being changed)
use the short locale ("en" and "fr") when dealing with language, this
way the framework's translations work correctly
add initial French translation (done by @Galixte)