Thanks for the pointer. I followed the link, and it says
The list of supported language classes is available in the class reference
The class reference it links to has a table of supported languages, and recognised keywords to select each one. I believe the “bottleneck” is happening on the discourse side, where it’s only passing a limited subset of language keywords to Highlight.JS.
In fact, it would be great if all the supported keywords were allowed, as some look quite interesting.
FYI, it has js for JavaScript, and c++ for C++. So I think this is definitely worth looking into.