@Rachel_Thorn Up until Mastodon v3.4, the language of a post was automatically determined on the server side. There were many misdetections for short posts of just a few characters, but for longer posts, it was usually correctly determined.
From Mastodon v3.5 onwards, the automatic language detection function on the server side was removed, and the language is now specified when posting.
Going forward, it may be better to add a function to automatically detect the language on the client side.