I understand the goal, and my comment wasn't so much arguing in favour of one language vs another, but rather that developing the same thing for multiple languages over and over seems wasteful.
In those client things the amount of the code that can be moved into a cross-language shared library might be lower than expected. E.g. the wire protocol and socket handling might be an option - but it's often not that big.
The bigger part might be transformation of the programming languages types into something useful for the client (e.g. through serialization), which has be be redone anyway for each language. And after that the question comes up whether sharing the remaining things yields enough benefits to justify the hassle of having a dependency which is less portable, requires another build system, etc.
That's my general experience with those kinds of projects - I don't know enough about RavenDB in particular to tell if it's the same here.
The goal of the project was to enable those Go programmers to be able to use RavenDB database in projects written in Go.
The company also maintains Python client library and Node.js client library.
It's not about Go vs. Java as a technology but enabling as many programmers as possible to use RavenDB.