The ABI is still C. Value types are just structs, and inheritance is implemented via aggregation - under the hood, it's all just interfaces and conventions on how they all play together. So any language that can do C FFI, can do WinRT. We did most of it with Python and ctypes as a prototype.

And you could even use that from C back in the day if you #included the "Windows.*.h" headers that shipped with the SDK - they had a bunch of #ifdef __cplusplus etc in them. Not sure if that stuff is still there or not.