When importing members, the country code should be based on the ISO 3166 Code. This is a 2 letter code that represents a country. So for example:
United States - US
United Kingdom - UK
United Arab Emirates - AE
Costa Rica - CR
Canada - CA
And so on..
If by mistake you would add a 3 letter code, only the first 2 digits will be taken into consideration. So if you were to add UAE for example for United Arab Emirates, it would actually show as Ukraine since Ukraine's ISO 3166 Code is UA:
You can find more information about the ISO 3166 code for your country in the following link.
Both the country code and the country name can be imported in the same CSV file.