somewhere in the serendipity_lang_en.inc.php....
wrong word is -> aditionally@define('INSTALL_EXTERNALSTYLE_DESC', 'You can define a stylesheet URI here, aditionally to the settings adjustable in the admin interface (enter \'none\' if you don\'t need this)');
and there are some duplicate entries there, why?? dont you want erase them?
Regards, omid