Mandarin, code cmn, is not yet translated.
Topic on Extension talk:Babel
We do not have cmn separately at this point. See https://translatewiki.net/wiki/Portal:Zh. We also have Gan Chinese [gan], Hakka Chinese [hak], Min Dong Chinese [cdo], Min Nan Chinese [nan], Wu Chinese [wuu], Yue Chinese [yue].
@Kaihsu: Nemo bis told me on phab:T117161 that that issue should be fixed on CLDR.