If you expect the majority of your donations/registrations to originate from a single country, set the Default Country for the mailing and payment address to that country in the Global Questions MailingAddress question. In order for both the payment and mailing address information to use the MailingAddress defaults, you need to use the MailingAddress global question.
NOTE: It is possible to have more than one global question with MailingAddress as a question name if you import campaigns on a regular basis. If you are not seeing the appropriate default country appearing on forms, verify that you have edited the correct MailingAddress question.
Change the default country in the Global MailingAddress question
Navigate to Library > Global Questions.
In the Global Questions section, click Edit to the right of MailingAddress.
In the Edit Global Question window, select the appropriate country from the Default Countrydropdown.
Click Save.