[Решено] Adding UK language

Background of the issue:
I am developing a site and trying to add the UK language in addition to the default generic EN language in the root domain. I followed an article on how to transform the Ukrainian language to UK but it doesn't work.

Symptoms:
The language name remains Ukrainian.

Questions:
How can I add the UK language in addition to the default generic EN language in the root domain?
Why does the language name remain Ukrainian after following the article?