Thank you for your job.
I tested this and I get PHP notice:
PHP Notice: Undefined variable: wgScriptPath in /srv/http/wiki/extensions/AddThis/AddThis.body.php on line 15
PHP 5.3.8 (FastCGI), MW 1.18.
Ruusian translation of the AddThis.i18n.php:
$messages['ru'] = array ( 'addthis' => 'Поделиться', 'addthis-desc' => 'Добавляет кнопку AddThis на боковую панель и в заголовок статьи.' );
Also I think you should add option to disable page header button.