+ get_preferred_language() should be able to set anyone's default language to their current browser language, if it's set. Totally untested, as tradition tells. (Class-System.php, Subs.php)
! loadConstants() was called a BIT too soon. The installer should be triggered before it, because it has yet to actually set the proper $boardurl. (index.php)

! Attempting to fix the installer: the thingy above, but also finding the proper paths for language files, and not initializing some things that were already set. Maybe this all makes it even worse... I don't know. (install.php)