I originally configured my site in 4 languages including en_US.

I want to add 'en_UK' to improve the original 'en_US' strings. I followed the docs (https://docs.osclass-classifieds.com/translations-i22#h3-3), especially the note:

Never edit en_US translation files (en_US folder). These are default files those will be overwritten with each update. If you want to just change english strings in your site, create copy of these, for example en_UK. To do this duplicate en_US folder, change it's name to en_UK and then edit all php/sql files in folder and replace en_US to en_UK.

But I can't find the 'English (United Kingdom)' language anywhere in the admin to select or activate it.

I also left the 'en_US' files in there for now.

In Admin > International > Languages > Add language it says:

You can import new languages and translations directly in Market > Languages section.

But Market > Languages (https://sexybelgium.be/oc-admin/index.php?page=market&action=languages) doesn' t show any 'English (United Kingdom)'.

What am I doing wrong?

Added en_GB here:
https://osclass-classifieds.com/translations

I uploaded it but error-message: "File uploaded but unable to activate the language"

Does not look like it was uploaded, try manually into oc-content > languages.

Still nothing...

Logged out and in again, cleared chache, etc..

Sorry, download again, name must be unique.
Then add it as zip.