Yes, you could map a custom field to a tag, you could create an acf/save_post filter outlined here http://www.advancedcustomfields.com/resources/acfsave_post/, get the field value and then add the term to the post using wp_set_post_terms(). This is essentially what ACF does for a taxonomy field. If I were setting this up I would more than likely use a custom taxonomy called “Country”, this is a perfect example of were it would be a good choice.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users