In Tags(1), choose an unassigned custom tag slot (probably the first one) and give it a name, maybe Country.
In the "save to music file as tag" box, choose Location. (I believe this is built in, but if it's not there, go to Define New Tags, enter "Location" in the first Identifier box, let it autofill the other fields, save and return to the main preferences, and then look in "save to music file as tag" again.)