Ideally we should get an autocomplete thing going there like emoji/mention, maybe one day…
You can see the list by typing Discourse.SiteSettings.highlighted_languages into dev tools.
"apache|bash|cs|cpp|css|coffeescript|diff|xml|http|ini|json|java|javascript|makefile|markdown|nginx|objectivec|ruby|perl|php|python|sql|handlebars|scss"
Clearly not something we expect you to do…