I'm planning on writing a C++ program which is heavily influenced by plugin modules. Initially I had only thought of making the APi available as an abstract class that others extend. After talking to someone they mentioned that I should think about making it available in a language agnostic way.

How do I effectively expose an API in a manner that any language can use it?

The solution that I could come up with was having the C++ API like I originally intended, for C++ directly and CLI languages (C#, F, etc) and then a sort of document and executable combination. Where expected input is described in the document and then the executable is started (via console/terminal) I would give parameters given as start up commands. This would theoretically work but doesn't sound like a standard solution too me. How is this done in other desktop software?