It's limited to .NET since it was easier for me to sandbox those bots. I've planned support for more languages in the future, but anything that compiles to .NET should be fine at the moment (https://en.wikipedia.org/wiki/List_of_CLI_languages).
I look forward to seeing this support additional languages.
I wonder if you have considered creating a remote interface for bots? If they are just sending and receiving json maybe something like websockets would allow for a language agnostic server without the need for you to worry about sandboxing different languages?