Use the #language #setting if you post in a different language, even occasionally. If your default is set to French, but you always post in English, every post when viewed by someone with a different interface setting will have a useless #Translate link that does nothing. French followers who don’t read English won’t get a Translate link.
You set your default language under Posting Defaults. You can override it post by post when composing a toot. You can also edit the toot later.
Screenshot of a toot translated from French to English
French post offering a Translate link in the English interface
Screenshot of post composition showing the language setting
Screenshot of Mastodon Posting Defaults showing Posting Language