var countries = ["GB", "US", "RU"].map(v => v.replace(/[a-z]/gi, match => String.fromCodePoint(match.toLowerCase().codePointAt() + 127365)));
var country = countries[Math.random() * countries.length | 0];
alert(country);
Или можно сразу объявить..
var countries = ["🇬🇧", "🇺🇸", "🇷🇺"];