This is a lot of code that's relatively complex given what you're trying to do.  There's a number of ways to do this, but off the top of my head:

* Just remove the option that's selected from the end_language 
* On change, set it to have the same options as the first one.  Just replace the html.
```javascript
  var selected_lang = $('#start_language option:selected');
  $('#end_language').html($('#start_language').html());
  $("#end_language option[value='" + selected_lang.val() + "']").remove();
```