From Instagram’s help page:
Captions and comments on posts in feed, as well as the bio you include on your profile, are translated automatically based on the language they’re written in and the language settings of the person viewing it. If translations aren’t showing up, it could be because we couldn’t detect or don’t currently support the language. Keep in mind that translations also may not show up for older posts and comments.
The big question: How accurate are the translations, particularly for medical terms? But since this feature is automatic, sounds like we’ll have no control over it. Something to watch.