Add missing translation for the downstream theme - zh_CN

Upstream changed between Havana and Icehouse, and one of the strings we
were relying on downstream is not present anymore in the upstream
message catalogues. A previous patch restored the string in Japanese,
let's do the same for Chinese from 6.0 also.

Related rhbz: 1122785

Change-Id: I2b7e5753768b988f6a5c3270061aa5c47acdbc95