I think crossing a language barrier here would be hard: you'd need a codegen step to compile the RPC spec to Typescript for the client-side DX and that would need take into account the full range of differences in serializability across the languages.

For cross-language RPC, I think we're still better sticking to a well defined system where the spec is data (OpenAPI, GraphQL, etc.) instead of the spec being code that only one side understands.