Add a drupal translation based on a already setup language?

Question

I have a site translated for a number of countries. Now I need to add a country but the language is already in the system (f.e. they speak French in both Belgium and France, or German in Germany and Austria). So instead of adding an extra language I'd like to have an url prefix for my new country but it needs to use both content and interface translation from an already existing language.

Any tips on how to approach this?