Technically it's only the C API that's important, the implementation behind the C API can be written in any language. The downside is though that you still need a language toolchains X, Y, Z to compile the implementations. Transpiling everything to C removes that one dimension from the build process.
Almost all non-C language offer that feature. The additional thing of the Zig toolchain is that it can also compile C/C++/ObjC projects without requiring a separate compiler toolchain, e.g. the Zig compiler toolchain is also a complete C/C++ compiler toolchain.
Almost all languages provide a method of calling C generated code for sure. But, as I understand it, this is usually a more tortuous path than straight ABI compatibility.
