Set the following values on your LocalSettings:
$wgLanguageCode = "zh";
$wgVectorShowVariantName = true;
This will make MediaWiki to display the name of the current variant above the menu.
On MediaWiki 1.17, there was an arrow to the right of the variant name, but on MediaWiki 1.18 this was changed to a double arrow which overlaps with the variant name.
See the attached screenshot.