Extensions can easily be built in any language, C++ is even documented, and I've built a simple one in Rust when I was learning it -- as long as you use the equivalent of extern "C" { } for that language, or rebuild the headers you need in another language (quite an undertaking, admittedly, even with a header parser).

The line "You will still need the C compiler that was used to build Python; typically Microsoft Visual C++." hasn't been accurate in a long time either (it's been in there since Python 2.2, when VC++ 6.0 was the latest, which had quite a few... quirks), anything that can digest the headers can link via the C API just fine. I guess since it's just not tested or supported, so you're on your own if you run into any issues with memory management, threading conflicts, compiler bugs, header conversion issues, etc.

-Em