Hah, I hadn’t even noticed that you could specify a language. Frankly the chat support for code is so bad that not getting an appropriate syntax highlighting seems like a secondary problem.
I mean Julia uses Slack officially and it doesn’t highlight code block at all… (if you use a snippet it does, but rarely you see people use that feature)
Here’s the official feedback forum, request to add Julia syntax highlithing (a couple years ago they had a different platform which had more votes on this topic): Add Julia syntax highlighting