[נפתר] Changing default key/code of the language

Background of the issue:
I want to add the region name in the subdirectory of my WordPress site using WPML. Specifically, I want to change the key/code of the standard language to include both the language and region code in the URL, for example, /en/gb/ instead of just /en/. I understand that I can create a new language, but I can't do it for the exact same language and region as the predefined ones. I am guessing that the only way is to somehow change the default implementation, possibly by updating the .mo file.

Symptoms:
I am unable to change the key/code of the standard language to include the region name in the URL.

Questions:
How can I change the key/code of the standard language to include the region name in the URL?
Is it possible to update the .mo file to achieve this?