Site only half translated into Chinese... HELP

I've been helping a new Drupal user setup his website via MSN Messenger, I've never worked with translations before and require someones help on a translation issue:

HE SAYS:
"I'm building my website on my home PC in Chinese and it works fine, but when I attempted to move the site to my online remote host it is only showing half the site in Chinese and the other half in English, so it has only half translated my site.

They only translated the core modules. If I download other modules from drupal website and unzip them to sites/all/modules, I need to translate the content of the new modules by myself.

I've tried changing the language from chinese to english and then back to chinese, but still showing half chinese and half english

If I download other modules from drupal website and unzip them to sites/all/modules, I need to translate the content of this new modules by myself. They never been translated. drupalTAIWAN website has some translation of some of the most popular modules, but only dozens of them. there are thousands of modules out there.

My website URL: http://www.i7491.com/

I'm happy to do the translation work myself, but don't know how to create a translation file for each module not in Drupals core!"