This seems like an odd engineering choice. Presumably the effort to create a python->go translator would be non-trivial. Why not just start rewriting components into Go, and migrating them out of Python, leaving python as essentially the presentation layer at most?
For a project the size of YouTube, that will be millions of dollars of engineering hours and weeks/months of lost productivity for an unknown gain and almost guaranteed bugs. It's a terrible value proposition so it's better to squeeze every last drop of performance out of the code base you have, which at the scale if this project includes paying engineer(s) to work on a completely new runtime.