<input name="lang" id="lang" type="text" />
<li id="ru" onclick = "addlang()">Русский</li>
<li id="en" onclick = "addlang()">Английский</li>
<script>
function addlang(e){
e = e || window.event;
var target = event.target || event.srcElement;
document.getElementById('lang').value += target.innerHTML;
}
</script>