Try to set the Language in the setting, Select Chinese and Save, English still there, setting not work, this should be a bug.
The Chinese translation is not yet approved on crowdin. Only approved translations land in Vikunja. As I have no knowledge of Chinese I’m unable to do that myself. If I would give you proofreading rights would you be able to approve them?
I would like to try, thanks! Email and username same as this community.
I see in crowdin all Chinese Simplified Language has been Translated and Approved, will it include in the next release? 
Chinese Simplified is completly translated but nothing is yet approved. Only approved translation strings will be included in Vikunja builds.
Checked and Approved, ready for launch 
Awesome! Translations are synced once a day so they should be available in the next unstable build tomorrow.