In reply to your fix you posted in language_initialize. It's very strange that the first arg would be empty.
So now that you've added in your changes, http://www.hillsongconference.co.uk/se/home works.
However, this should be exactly the same as http://www.hillsongconference.co.uk/?q=se/home
and you will see that this brings in the 404 error (whereas q=/se/home doesn't).
So I'm worndering if the htaccess file has been edited? As this is how Drupal converts the arguments in the url path to
$_GET['q']. The Drupal .htaccess file also strips out leading slashes, to insure the first argument isn't empty.
You've hacked core now, which is a terrible no-no. Even if you are never going to update this site, it still needs to be fixed out of principle!
If you are running a nginx setup the correct rewrite (to not include the slash) is
rewrite ^/(.*)$ /index.php?q=$1 last; not
rewrite ^(.*)$ /index.php?q=$1 last;