Yeah, that’s exactly what I meant. But I get that you want to avoid HTML if possible.
If you find a pure markdown alternative, let me know, I was looking at options a while back for a repo and settled on the HTML tag.

Github accepts the details HTML tag as part of Markdown. You could use that to put there a description and it would only be visible if you expand it.

I’ve failed to get that to work. How do you actually get additional languages?