No, I just found out about Pliant from your post - but it doesn't make sense for developing large, stable codebases because it's not a standard language. (Sorry Pliant developers - I love your can-do attitude and I'd love to buy you a beer or a coffee sometime and talk about Sisyphean task management.) But Pliant is a reference implementation of a custom language. Programming language design is really, really hard - I wouldn't dare try and so I chose to go with a language that had literally hundreds of person years of design and testing underpinning it.
Regarding FFI's - my approach is the same as the very clever pybind11, luabind and the older boost::python libraries. It works by using C++ template programming and letting the C++ compiler do the heavy lifting of generating wrappers at compile time. I recently updated our binding library to use C++17 std::apply with tuples. Freakin' finally! C++ has an apply construct that can be applied to a heterogenous list of objects - wow - lisp has only had it for 50 years! My point is that only recently has C++ developed the introspective capabilities to implement really powerful FFI's. Also - you have to use C++ exception handling for stack unwinding or you will break C++ RAII all the time.
What 'drmeister means is that CL as an actual standard, as in ANSI standard. Ditto for C++ (an ISO standard). This means the language has a clear target for implementations to conform to, one that isn't vulnerable to "moving fast and breaking things".
