It's based on Monaco Editor, which is the open source base of VSCode - you can see the full list of supported languages by clicking the dropdown on the bottom right (similar to VS code: https://code.visualstudio.com/docs/languages/overview#_chang...)

Actually, it's based on CodeMirror

The above is a partial list, Edna supports about 30 langs.

To add another one, you need to find library that implements CodeMirror mode for that language, add it to package.json and update languages.js