The interface language, and where to change it

All thirteen languages in Settings › General › Language are translated, and picking one moves every label, tooltip, dialog and status message together. Twelve have a catalogue of their own; Traditional Chinese is converted from Simplified rather than written out twice, so it arrived complete at the same time as the rest. Dates and numbers format in the language you choose. The first launch follows your Mac.

This is not the same setting as the language of the call. The interface can be in English while the transcript is Chinese, and neither one moves the other.