I've had a look into this myself in the past. It seems Magento sets the Default Language as a Constant inside the following class:
Magento\Deploy\Console\Command\DeployStaticContentCommand
/**
* Default language value
*/
const DEFAULT_LANGUAGE_VALUE = 'en_US';
It then references the constant when it prepares the deployable entities, inside private function prepareDeployableEntities($filesUtil)
.
So without going into too much detail, maybe you could write a preference for your own class and set the constant to en_GB
.
See the following for some good instructions on how to set a preference in M2:
http://devdocs.magento.com/guides/v2.0/extension-dev-guide/plugins.html
Your extensions etc/di.xml
would looks something like below:
<?xml version="1.0" encoding="UTF-8" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<preference for="Magento\Deploy\Console\Command\DeployStaticContentCommand" type="Namespace\Module\Console\Command\DeployStaticContentCommand" />
</config>
Another option could be to redefine the argument for staticContentDeployCommand
although I'm not too sure how this would work. Possibly just like below in your etc/di.xml
:
<type name="Magento\Framework\Console\CommandListInterface">
<arguments>
<argument name="commands" xsi:type="array">
<item name="staticContentDeployCommand" xsi:type="object">Namespace\Module\Console\Command\DeployStaticContentCommand</item>
</argument>
</arguments>
</type>
Hope this helps,
Josh