If you encounter the "Fatal error: __clone method called on non-object in ..." error
The following error message or a similar message might appear while configuring WPML:
Fatal error: __clone method called on non-object in /home/.../wp-content/plugins/sitepress-multilingual-cms/sitepress.class.php on line 2958
In this case, you can resolve this problem by registering the WPML plugin by adding the site key.
If you ignore the above error message and continue to configure WPML, you might see another message on your site and your site might be broken (and the database might also be corrupt):
Warning: mysqli_set_charset(): Error executing query in /home/.../wp-includes/wp-db.php on line 782
The error message "Fatal error: __clone method called on non-object in..." occurred when I tried to add a menu for a different language. But when I registered the site key in advance, no error message appeared.
WPML is a WordPress plugin which converts WP sites into multilingual sites without any hassles.