zh
zh-classical
zh-cn
zh-hans
zh-hant
zh-hk
zh-min-nan
zh-mo
zh-my
zh-sg
zh-tw
zh-yue

but the better list: https://www.wikidata.org/wiki/Help:Wikimedia_language_codes/lists/all   it is complicated because :  zh-yue = yue ;  zh-min-nan = nan ;  zh-classical= lzh  

>But apparently there is no such values as 'zh-cn', 'zh-nans' etc.

I see "zh-cn" and "zh-hans" labels..
example:  Q2 Earth: https://www.wikidata.org/wiki/Special:EntityData/Q2.json

,

"zh-cn": {
"language": "zh-cn",
"value": "地球"
},
"zh-hans": {
"language": "zh-hans",
"value": "地球"
},
"zh-sg": {
"language": "zh-sg",
"value": "地球"
},
"zh-hk": {
"language": "zh-hk",
"value": "地球"
},
"zh-tw": {
"language": "zh-tw",
"value": "地球"
},
"zh-mo": {
"language": "zh-mo",
"value": "地球"
},

or a programmer example:

# curl -s https://www.wikidata.org/wiki/Special:EntityData/Q2.json |  jq '.' | grep language | egrep 'nan|yue|lzh|zh-'