I meant "what functionality is the remaining 5%." Not "what language is it."

Basically, once you are 95% Go, what parts are hard to convert to Go to make it 100%

Probably binding to pre-existing C libs.

It will take some time to replace 40 years of existing accumulated C libraries.

That would be silly. It's silly to talk about the source language of third party libraries when discussing languages you use. 3rd party library code is all object code or bytecode for all it matters to your project.