In the official Mastodon web app, the language selection menu in the compose window is the worst design affordance-wise. Nobody, unless they're a nerd, would ever look at the two capitalized letters (e.g., ZH for Chinese) and assume that they could choose a language.
(Some would say that the ISO 639-1 language code is “not an obstacle,” any more than phone prefix numbers are, but, well, who remembers phone numbers these days?)
https://github.com/mastodon/mastodon/issues/17073#issuecomment-1112038012