Are other countries besides the United States (US) allowed for credit card payments?

Yes, all countries can potentially be added to the billing and shipping address dropdown on the checkout form.

The United States only is selected as a default. In order to accept purchases from other countries, you will need to make sure to select all of the valid countries you want to accept payments from. Here is a video and article to help you get those set up: Setting Valid Countries for Checkout Forms.