How to use value from modal tinymce windowManager?

If I read your question correctly, this is more related to Javascript programming than WordPress.

To get the selected option value or text of a HTML select tag, use:

var e = document.getElementById("programming-language-id");
var value = e.options[e.selectedIndex].value;  //your code doesn't set option value
var text = e.options[e.selectedIndex].text;  //this will get the selected option text