@rotto since Mastodon does not only have 1 single user interface, it really depends on which app you are using. When in doubt always try accessing Mastodon via a web browser, because the browser user interface will have all the latest features.
And yes, if you visit my profile through a web browser, you will see a "Translate" button underneath my Danish toots. Well actually, it is just a link, and it looks like this: