It is difficult to display multiline selections in combo textbox. An alternative way to solve this issue is to place a multiline component beside of the combotree and put the selected values into this multiline component when users select a tree node.
Here is the simple example:
<div style="position:relative">
<textarea id="ta" readonly style="position:absolute;left:0;top:0;width:176px"></textarea>
<select id="c2" class="easyui-combotree" name="language" data-options="onCheck:onCheck,url:'tree_data.json',cascadeCheck:false" multiple style="width:200px;"></select>
<script>
function onCheck(v){
var s = $('#c2').combotree('getText');
$('#ta').text(s);
}
</script>
</div>