This would only work if your visitors (even the ones not logged in) will accept cookies. You could then have an "english" and a "chinese" theme (because of the buttons you mentioned) and let users switch language and theme by clicking on a button or link which would then set the visitor's cookies for language and theme accordingly.
You could then modify index.php to pull the english or chinese stories from the database, according to the language set in the cookie.
If you want to make this work without cookies, you would need two (more or less) complete installs of Geeklog ...
bye, Dirk