I highly recommend setting the language of your posts, as this helps people using screen readers. The content is automatically read with the correct voice and pronunciation. This feature of setting the language of your post isn't implemented in all apps yet, but works well in the web interface.
If you only post in one language you can set a default (but do remember to set that default if it's not English).
Also, it helps people filter posts from specific languages.
If you aren't seeing this setting, it could also be the case that your Mastodon instance hasn't updated yet. Talk to your admin :)