There are two different but confusingly named extensions for syntax highlighting:
SyntaxHighlight extension, formerly known as
SyntaxHighlight_GeSHi—the extension to which you linked—uses server-side highlighting via Pygments. That one is not installed on imagej.net (!).
When I installed the GitHub extension in December 2016, it did not support syntax highlighting, nor even verbatim code blocks. Therefore, I forked the GitHub extension to support syntax highlighting via
SyntaxHighlighter; see imagej/mediawiki-GitHub@1637d87e.
Since then, there have been dozens of new commits to the
GitHub extension upstream, including the addition of syntax highlighting via the
SyntaxHighlight extension. However, upgrading to the latest version of
GitHub might be disruptive. We might need to switch to—or at least enable—the
SyntaxHighlight extension, and when I tried to do that in the past (during the server rebuild of summer 2016), I could not make it work, which is why we stuck with
SyntaxHighlighter even though as a MediaWiki extension it seems dead (no commits in over two years).
Note that this is not the first time we have desired more languages. To address that, we forked SyntaxHighlighter and added more brushes. The weird thing this time is that there is already a
shBrushGroovy.js. There was not, however, an alias mapping
Groovy, so I added one. And it seems to have done the trick.
Thanks for the report, and please let me know if you notice any other missing languages!