In class.Language.inc.php, function readUserHeader a notice will be reported if:
- error_reporting(E_ALL); has been called and
- a proxy like: http://anonymouse.ws is used

Error-No. 8
Error-Type: Notice
Undefined index:  HTTP_ACCEPT_LANGUAGE

File: class.Language.inc.php in Line 566 (version 1.061)

Bugfix:
Add this before first line of function:
if (isset($_SERVER['HTTP_ACCEPT_LANGUAGE']))
{