Add Simplified Chinese (zh_CN) language selector option

Summary:
Tyson Tan and Aster Wang are actively translating Krita documents. By adding
option to preview the website in Simplified Chinese, their work will be easier
more efficient. Thanks!

Reviewers: Krita: Manual, woltherav

Reviewed By: Krita: Manual, woltherav

Subscribers: woltherav

Tags: Krita: Manual

Differential Revision: https://phabricator.kde.org/D19689