Removing required status from checkout fields fails when a locale is active

[f1-outsourcing]

I have found some info on how to remove the required status from checkout fields and changed these files.

/app/design/frontend/rwd/xxxxxx/template/persistent/checkout/onepage/billing.phtml
/Mage/Customer/Model/Address/Abstract.php
/Mage/Eav/Model/Attribute/Data/Text.php

This results that in store views with en_GB or en_US locale the checkout is indeed working as expected.

However when I created a new store view and change the locale to eg. es_ES and copy the locale files to the /app/locale/ folder. I am getting a js popup complaining about required fields.

There are no .js files in these /app/locale/xx_XX folders, so is this a bug? How to fix this?