I have installed the Forum Plugin into my OS-Class site. However, my default language is Chinese Simplified and how can I add a Chinese language pack to the plug in. I do see a en_US sub folder under the languages sub folder of the plug in. Can I just copy it, and rename to zh_CN and add the translate phase to the msgstr quote? 

Check this:
https://osclasspoint.com/blog/how-to-translate-themes-and-plugins-b3