why I can not change the default UI language of DropEditor toolbar?

When I testing with DropEditor Free version on my computer, it always use "zh" as the default UI language for those toolbar buttons. Since I speak Chinese Simplified (zh-cn), not Chinese Traditional (zh), so I want to change the default language of the UI.

Well, it seems that DropEditor uses CKeditor as the core. So I followed CKeditor's documentation, modified its config file :

/plugins/editors/dropeditor/ckeditor/config.js

And insert following code:

<pre class="line-numbers"><code class="language-markup">config.defaultLanguage = 'zh-cn';
</code></pre>

Then I save that file and clear cache. Next I tried to re-open the article editing page of Joomla 3, but the editor there still uses "zh" as the UI language.

How can I change the default UI language of editor toolbar?

Thank you.