This Mastodon setting is really important:
1. Log into your server's website
2. Go to ⚙️ Preferences (or Settings)
3. Click "Posting Defaults" (on mobile click ☰ in top right, then "Posting Defaults")
4. Make sure "Posting Language" is same as language you post in most often
5. Click "Save Changes" at top
This matters because many people use language filters to only show posts in their language. If your setting is wrong, your posts may be hidden from people who can understand them.
p.s. If you post in more than one language, set the default to be the one you use most often.
If you ever make a post in a different language that you don't usually use, you can manually change just that post's language setting in its post creation box.
When you are creating a post on the web interface you will see the language's full name (e.g. "English"), while on the apps you may see a language's abbreviation (e.g. "EN"). Click on this to adjust just that post's language setting.