Go has a second compiler—gccgo. Python also has other implementations, but because there is no Python standard, the compatibility of those implementations with the broader ecosystem varies enormously and CPython is the de facto standard (the extent to which an implementation wants to be compatible is the extent to which it must be slow, because it means adhering more closely to the design mistakes that CPython has committed itself to).

Python is a bad example. Cython and pypy are very widely used. IronPython, Jython, etc. also exist. Go is a better example, but even Go has a gcc backend, although it is rarely used in production as I understand.

Go has a spec. Rust doesn't even have a spec at the moment.