C++ doesn't really have a "backend" like the JVM does; its ABI is already too weak to be used across shared libraries. So there's not as much need to mix a new language with existing C++.
Worse, because C++ libraries are gigantic header files, you have to support 100% of the language to use them.
It's still possible, and it has been done before. One of the most amazing examples is Clasp [0] - a Common Lisp implementation that is able to fully interop with C++, including template code and exceptions. And it's led by a Chemistry PhD - I'm sure someone like Herb Sutter can run miles around him in programming expertise.
https://github.com/carbon-language/carbon-lang