Add a new language with locale , eg. Us English and UK English

We would like to have our store available in two different languages, US English and UK English. The product descriptions would be different for each of those and some products would be available on only one of the stores.
Right now i can only see the possibility of adding multiple languages but no option to do something like the above. What would be the best approach to implement something like that?

This link will help with product appearing for certain locales, but changing the webpage language to british english for UK customers seems not possible as of yet.

https://www.shopify.co.uk/partners/blog/customize-content-by-country