Hi, I have a question…

Hi, I have a question about the use of localesLocale Locale = language version, often a combination of a language code and a region code, for instance es_MX denotes Spanish as it’s used in Mexico. A list of all locales supported by WordPress in https://make.wordpress.org/polyglots/teams/. Specifically I wanted to know why the team decided to support en_gb against en_uk coding standard. I am a translator approaching php and that confuses me. As a translator I never use country code en_gb to identify English but en_uk. I have seen php doesn’t support en_uk also. Is there a reason for this or is it just a choice dictated from what works and what not?
Thanks for your kind answer.