function letterSort(lang, letters) {
letters.sort(new Intl.Collator(lang).compare);
return letters;
}
console.log(letterSort('de', ['a','z','ä']));
// expected output: Array ["a", "ä", "z"]
console.log(letterSort('sv', ['a','z','ä']));
// expected output: Array ["a", "z", "ä"]