Plain

var firstSelect = new birthdaySelect({
  id: '#birthdaySelect-1',
  lang: "de"
});

firstSelect.init();

Bootsrap

var secondSelect = new birthdaySelect({
  id: '#birthdaySelect-2',
  bootstrap: true,
  labels: ['Day', 'Month', 'Year'],
  lang: "en"
});
secondSelect.init();

/*

	BOOTSTRAP VALIDATION

*/
window.addEventListener('load', function () {
  var forms = document.getElementsByClassName('needs-validation'); 

  var validation = Array.prototype.filter.call(forms, function (form) {
    form.addEventListener('submit', function (event) {
      if (form.checkValidity() === false) {
        event.preventDefault();
        event.stopPropagation();
      }

      form.classList.add('was-validated');
    }, false);
  });
}, false);