After a few weeks there are some group qualifying matches with your Internation team. By playing well for your club team, you'll get chosen to play some of these matches (beginning as a substitute). After you play your first match for your national team the achievement should unlock.
Be aware that when you create a Become a Legend player you must choose a country that competes in the Internationl Cups (if you've choosen a country that doesn't compete then you'll get a message, before creating an account, that you can't play for your national team).