I tried to clone the Zone Based Rates Shipping Module but it does not work properly in the front-end.

The shipping module can be installed, it shows up at the back-end, and I was able to configure it.
But on the front-end, the Checkout Shipping does not continue to step 2 (it remains in Step 1 of 3 - Delivery Information).

I followed the steps Ajeh described in http://www.zen-cart.com/forum/showthread.php?t=26216
and copied the \includes\modules\shipping\zones.php to zones_es.php
and changed
Code:
class zones {
to
Code:
class zones_es {
and
Code:
function zones() {
$this->code = 'zones';
to
Code:
function zones_es() {
$this->code = 'zones_es';
and I replaced all occurences of
Code:
MODULE_SHIPPING_ZONES_*
to
Code:
MODULE_SHIPPING_ZONES_ES_*
(e.g. MODULE_SHIPPING_ZONES_TEXT_TITLE to MODULE_SHIPPING_ZONES_ES_TEXT_TITLE).

Furthermore I also cloned & changed the language file, and those labels are shown correctly at the front-end.

What did I do wrong? What did I forget during the cloning?