I think (but I am not fully sure) to have found a problem in setup.php which could lead to an undefined $wgContLang object under certain circumstances; pls. correct me if I'm wrong.

The problem can be solved by executing loadfromsession() some lines later, i.e. after the language settings. Would be nice, if anyone can have a look to it.

[1] http://bugzilla.wikimedia.org/show_bug.cgi?id=1805 (for version 1.4)