I'd try clearing out the cache/forum_cache.php file manually and then rebuild. Basically, at the end of that file it has an array that should look something like this:

Code
$lang_cache = array (                                                          
  1 =>                                                                         
  array (                                                                      
    'dir' => 'english',                                                        
    'name' => 'English',                                                       
  ),                                                                           
);

It sounds like yours isn't built properly. Deleting the file and then clearning cache should properly build the array.