For app versions 1.14.0 +
custom brushes for additional languages are not supported anymore due to technical problems in combination with other apps not provided by us.
For app version up to 1.13.5
We support custom syntax highlighting languages in the app, up to version 1.13.5.
You can add support for custom programming languages in the Confluence admin area:
General configuration -> Configure Code Macro -> Add a new language
You need a so called "brush" file for your new programming language. See this page about the format of these files and where you can find existing brushes.
After you have done this, you should be able to see your new programming language in the list of supported programming languages in the plug-in's macro configuration dialog.