And this to enable default language to work.

functions/i18n.php

@@ -702,8 +702,10 @@ function set_up_language($sm_language, $do_search = false) {
     if ($do_search && ! $sm_language && isset($HTTP_ACCEPT_LANGUAGE)) {
         $sm_language = substr($HTTP_ACCEPT_LANGUAGE, 0, 2);
     }
+    
     if (!$sm_language && isset($squirrelmail_default_language)) {
         $squirrelmail_language = $squirrelmail_default_language;
+        $sm_language = $squirrelmail_default_language;
     }
     $sm_notAlias = $sm_language;
     while (isset($languages[$sm_notAlias]['ALIAS'])) {