We eventually plan to support all features in all languages, but we have no timeline. A lot of it is made by external contributors. Your help would definitely be welcome!
Some languages in gRPC are supported directly in the C++ implementation (like Python), others have their own stand-alone implementation (like Go), so the effort to support them depends on which direction this goes. Typically, to support a language you first need to make a PR to gRPC itself (to make it not require Protobuf, most of this work has already been done), then add a code-generator for the language to FlatBuffers.
Examples of what this looks like you can find in my links from here: