Sorry for the poor English, it is not my first language. As is, LanguageSelector doesn't automatically convert page contents to Chinese variants using the built-in ZhConversion.php. I added the following line below
$user->setOption( 'language', $requestedLanguage ); in LanguageSelector.php:
$user->setOption( 'variant', $requestedLanguage );
Now the extension should convert the page content between Chinese variants, in addition to the UI elements. In theory it should also work for other languages with built-in language conversion.