code: function initializeLocaleList(locales) {
    var seen = new InternalArray();
    if (!IS_UNDEFINED(locales)) {
      // We allow single string localeID.
      if (typeof locales === 'string') {
        %_Call(ArrayPush, seen, canonicalizeLanguageTag(locales));
        return freezeArray(seen);
      }
  
      var o = TO_OBJECT(locales);
      var len = TO_UINT32(o.length);
  
      for (var k = 0; k < len; k++) {
        if (k in o) {
          var value = o[k];
  
          var tag = canonicalizeLanguageTag(value);
  
          if (%_Call(ArrayIndexOf, seen, tag) === -1) {
            %_Call(ArrayPush, seen, tag);
          }
        }
      }
    }
  
    return freezeArray(seen);
  }
docs: Returns an array where all locales are canonicalized and duplicates removed.
  Throws on locales that are not well formed BCP47 tags.