Mastodon has a built-in translation system for posts in other languages. For it to work, three things have to happen:
- Your server has to offer the service
- Your language settings have to be correct
- The person posting has to use the correct language settings
If all these are ok, you will see a "Translate" link below the post or in the post's ⋯ menu when you encounter a post in a foreign language.
Lots more info and questions answered at:
➡️ https://fedi.tips/is-there-a-built-in-language-translation-system-on-mastodon