Hi @wloske
You can always add the script by using the acf/input/admin_footer and then add a new text field to hold the country name. Please take a look at this page to learn how to work with ACF and Javascript: https://www.advancedcustomfields.com/resources/adding-custom-javascript-fields/.
If you still want to have the country field added to the core files, please open a new ticket so your suggestion can be passed directly to the plugin author. You can open a new ticket here: https://support.advancedcustomfields.com/new-ticket.
I hope this helps 🙂
