Little headsup, Mastodon developers seem to be testing a language selector in the posting interface (see attached image).
The idea is you set the language you're posting in which will make it easier for other people to filter your post out if they don't speak it.
You can already set your default language in the Preferences/Settings section, but the idea of this new feature is to let you optionally change the setting for each particular message. This is handy if you toot in several languages.
I have no idea if/when this will be rolled out, the developers' instances often get test features like this.