Set Value Inside a TinyMCE Editor using jQuery

I think you can do:

$(function() {
    $("#lang").change(function() {
        var s = $(this).val(); 
        alert(s);
        tinyMCE.activeEditor.setContent(s);
    });
});

Simply this works for me

$("#description").val(content);

For me only that's code works :

tinyMCE.get('my_textarea_id').setContent(my_value_to_set);

Maybe this is the code from the new version of tinyMCE ! (Tiny MCE Api 3)