You can't highlight a language with Markdown. You need to outsource this to external libraries. Such as.
EDIT: The StackOverflow way of doing this is to have a comment at the top. Like so.
<!-- language: lang-js -->
Just replace lang-js with one of the language codes or a tag to set the language to the language associated with that tag. For example.
<!-- language: C# -->
Highlighting for C# is associated with the C# tag.
