View of related nodes by continent or country without taxonomy terms
There are like mostly in Drupal several possible solutions. One would be to filter Views views for each view and therefore you can filter any field you want the same way, like you do with terms (check the terms view pages as an example, how to build such filtered views).

Another option would be to create node relations like provided with the Relation module or the Entity reference module. The advantage of the relation module is, that the relations are own custom entities which are fieldable again, like term view pages. Thanks to @diqidoq for pointing me to this solution.