Rather than choosing a scripting language for mods/plugins/extensions, you can now choose "Well, all of them!" Currently five guest languages are supported in thirteen host languages, but more are coming. It's mostly other peoples' job to make sure more are coming, but if your preferred guest language is not yet supported and if your use case requires sandboxing or any kind of permissions management (which is what makes this harder than just using the FFI), it might be about the same amount of work to roll your own plugin architecture as it would be to add support for that language to Extism. If you add the language to Extism, you also get all the other guest languages for free, and also all the other Extism users can then use the language you added support for.
See appcypher's list of projects to make various languages work in WebAssembly, which is the first and largest step in making a new guest language work in Extism. I'm most looking forward to Python and Lua guest support, both of which already have working WebAssembly prototypes (python demos: cheimes, pyodide).