Here's an approach that should let you hack additional language brushes into the preview of 5.1.10. It modifies one of the shadow tiddlers within the highlight plugin, which can cause problems with future upgrades; if that happens, remove the shadow tiddler override and then re-apply the technique described below.
2. Copy the text of the language you want
3. Open the tiddler "$:/plugins/tiddlywiki/highlight/highlightblock.js"
4. Paste the following after the line "hljs.configure(...)":
hljs.registerLanguage("<name_of_language>",<text_of_language_file>);
Save the tiddler and restart.
Let me know how you get on,
Best wishes
Jeremy.