The Database Language Locale setting will determine which country is selected by default:
  1. Click Management in the menu
  2. Click My Organization
  3. Click the blue Preferences link in the account header
  4. The Database Language Locale field can be found under the Preferences section:
Database Language Locale

This setting cannot be adjusted within the database; if your locale is incorrect, please Contact Support and reference this article.

NOTE:

It's also possible that you have some defaults set on the New Account screen which determine what Country is selected by default. You can clear out your default settings using the steps below:
  1. Click Accounts in the menu
  2. Click Add an Account
  3. Click the Clear Defaults button on the left hand side