As you may (or may not) know, Address Book in OS X lets you specify a country format for the Address field in each contact’s record. To specify the country format for a given entry, select that entry in Address Book’s list of contacts, then click Edit. Click the word Work or Home next to that contact’s address, and a drop-down menu will appear. The last entry in the drop-down is Change Address Format; move your mouse over this entry, and a huge list of countries appears. Select the country you’d like to use for that contact’s address format, and you’re done…sort of.
If you’re dealing with imported records (or records that you entered yourself) that lack an entry in the contact’s Country field, you’ll notice that the Country field is still blank after you select an address format for that contact. So annoyingly, after selecting the contact’s country in the drop-down menu, you’ll then have to go back and type it in to the Country field to finish your edits.
There is, however, a bit of a shortcut for this process. As it turns out, if there’s any text in the Country field, then Address Book will replace it with the country you choose in the Change Address Format menu. So before you use the drop-down menu to set the address format for a contact, just type something—like a single a or b or whatever—into the Country field if it’s blank. When you then select the address format, the Country field will reflect that same selection.
It seems to me that this feature should work even if the Country field is blank, but at least there’s a minimal-typing-required workaround.