Try K
Settings
Control center
Regional Access
Country Regions and language
country
add language

if it is not there download it
you must be connected and then
run in a console
apt-get update
apt-get install kde-i18n-zhcngb2312

return Control Center. Chinese simplified must be there.

apt-get is easy with synaptic (a gui frontend)

apt-get install synaptic

then run
synaptic