On Wikidata, the termsbox (that contains item labels, descriptions and aliases) shows a selection of languages (via ULS?), and users can customise this by adding a Babel list on their user page.

If you add a Babel entry for, say, zh-sg or zh-tw, they show on the page and in the API normalised to zh-Hans-SG or zh-Hant-TW. (The normalised codes match those in the drop-down for interface language in User Preferences.)

However, on Wikidata, the termsbox/termbox(?) does not show Singaporean or Taiwanese labels as a result.

See

• Languages available "Traditional Chinese" and "Chinese"? – discussion at Wikidata
• T101086: Standardize invalid language codes for Babel extension

I don’t know whether this is relatively unimportant: are the national varieties needed much if Simplified and Traditional both work?