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.
Zig has been in the news a lot recently but I haven't explored it much other than a few tech talks which seemed interesting.
I wonder if this is it's killer feature - compatibility with the C ABI.