For regular languages (not templating), if they have good tooling for converting to/from AST it's quite possible. The codebase isn't very flexible right now, but it's also not very big. In fact monocle itself is less than 900 lines of code.

I'm not sure if it makes sense for monocle to support multiple languages, or if each language should have its own port. Someone writing monocles in a specific language probably wants to do custom scripting in the language they're used to.

Templating languages are trickier, because they usually don't have great tooling. In fact, we wrote our own tooling to convert from ERB to builder (a template system where you generate HTML through Ruby methods) and back. So for any templating language you would probably write a tool that converts from that language to builder, and back.

On the other hand, ERB is about as free-form as it gets. Templating languages that are more strict are probably easier to add. For more info on how ERB is supported, I wrote another post that goes into detail: https://blog.luitjes.it/posts/erb2builder/

If there is an existing parser, like for a lot of templating languages, this is can be done, since templates are parsed to ruby in a separate step. Otherwise, it would require creating a ruby-based parser for a language, or porting the concepts to that language.