<div id="dropdown-menu" class="dropdown-menu">
  <a class="title">Валюта: USD</a>
  <ul>
    <li><a data-language="usd" href="#">USD</a></li>
    <li><a data-language="euro" href="#">EURO</a></li>
    <li><a data-language="shekel" href="#">Шекели</a></li>
  </ul>
</div>

var languages = {
 usd: 'Валюта: USD',
 euro: 'Валюта: EURO',
 shekel: 'Валюта: Шекели'
};

var DEFAULT_LANGUAGE = 'usd';
var menuElem = document.getElementById('dropdown-menu');
var titleElem = menuElem.querySelector('.title');

function selectOption(optionName) {
 titleElem.dataset.language = optionName
 titleElem.innerText = languages[optionName]
}
selectOption(DEFAULT_LANGUAGE)
titleElem.onclick = function() {
 menuElem.classList.toggle('open');
};
var optionsHolter = menuElem.querySelector('ul')
optionsHolter.onclick = function (event) {
  menuElem.classList.remove('open');
  console.log(event.target.dataset.language)
  if (event.target.dataset && event.target.dataset.language) {
    selectOption(event.target.dataset.language)
  }
}