It depends on your instance, the admin has to pay for an integrated translation service (usually an open source one such as LibreTranslate).
Ask your admin if they have this activated on your instance.
As Matte says, if the service is active there will be a "translate" button below posts in other languages.
(Note that for this to work correctly, you have to set your own language settings correctly, more info at https://fedi.tips/setting-your-language-preferences-on-mastodon-and-filtering-out-posts-in-other-languages/)