Replace hardcoded message by localized string

Fixes dokuwiki#4507

Author: dregad

lib/plugins/usermanager/admin.php

@@ -722,7 +722,7 @@ protected function deleteUser()
         $selected = array_keys($selected);
 
         if (in_array($_SERVER['REMOTE_USER'], $selected)) {
-            msg("You can't delete yourself!", -1);
+            msg($this->lang['delete_fail_self'], -1);
             return false;
         }
 

lib/plugins/usermanager/lang/en/lang.php

@@ -41,6 +41,7 @@
 $lang['nonefound']   = 'No users found. %d users total.';
 $lang['delete_ok']   = '%d users deleted';
 $lang['delete_fail'] = '%d failed deleting.';
+$lang['delete_fail_self'] = 'You can\'t delete yourself';
 $lang['update_ok']   = 'User updated successfully';
 $lang['update_fail'] = 'User update failed';
 $lang['update_exists'] = 'User name change failed, the specified user name (%s) already exists (any other changes will be applied).';