How to populate the options of a select element in javascript

You can create the option inside the loop;

for(element in langArray)
{
   var opt = document.createElement("option");
   opt.value= index;
   opt.innerHTML = element; // whatever property it has

   // then append it to the select element
   newSelect.appendChild(opt);
   index++;
}

// then append the select to an element in the dom

You need to create your option element inside your loop, set attributes and text and append it to the select element:

var select = document.createElement('select'),
    option,
    i = 0,
    il = langArray.length;

for (; i < il; i += 1) {
    option = document.createElement('option');
    option.setAttribute('value', langArray[i].value);
    option.appendChild(document.createTextNode(langArray[i].text));
    select.appendChild(option);
}

This assumes that your langArray looks something like this:

var langArray = [
    {value: "val1", text: "text 1"},
    {value: "val2", text: "text 2"}
];

You'll need to tweak the code to match your array