This is fantastic and highly addictive! My only suggestion (as others have said) would be to indicate the CORRECT flag's name clearly and give the user a chance to absorb it before going onto the next one. Maybe a box around the right choice or the country name near the flag? Something like that.

The design does make me want to "get 'em all" and learn them correctly and that little change would help.

Kudos on a fantastic first app which is very professional and usable!