var lang = {
    HELLO: 'Привет'
};

document.write(lang.HELLO);

LAT : ["0", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX", "X", "XI"]

<select onchange="changeLang(this)">
  <option value="romanNumerals">Римские цифры</option>
  <option value="arabicNumerals" selected>Арабские цифры</option>
</select>

<div id="digits">1, 2, 3, 4, 5, 6, 7, 8, 9</div>

<script>

var dictionary = {
  romanNumerals: [ "0", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX" ],
  arabicNumerals: [ "0", "1", "2", "3", "4", "5", "6", "7", "8", "9" ]
};


var prevLang = document.querySelector("option[selected]").value;
var separator = ", ";

function changeLang(element) {
  var currentLang = element.options[element.selectedIndex].value;
  
  digits.innerHTML = digits.innerHTML
    .split(separator)
    .map(function (digit) {
      var index = dictionary[prevLang].indexOf(digit);
      return dictionary[currentLang][index];
    })
    .join(separator);
  
  prevLang = currentLang;
}

</script>