Style-specific language variables

Discussion
RFC

Basically, the idea is to give style authors the ability to create new style-specific sections, such as a "Quick Links" area without having to hard code the language or wait for new language entries to be added into phpBB core.

Implementation: In user class, setup method (of includes/session.php) look for and include the following files in styles/%currentstyle%/lang/:
-board default language
-user selected language (if different from board default language)

These files, while shipped with the style and residing in the styles directory, are not tied to a specific style and can be copied into other style directories for use with other styles, and can be modified per language, per style to suit the needs of the board management.