Go and Dart are probably the languages most likely to work well with gRPC, given their provenance.

Google has massive amounts of code written in Java so one would think the Java tooling would be excellent as well.

Doesn't Google mostly use Stubby internally, only bridging it with gRPC for certain public-facing services?

Google also uses a completely different protocol stack to actually send Stubby/Protobuf/gRPC around, including protocols on the wire and bypassing the kernel (according to open access papers about PonyExpress etc)