Does an actor system need to be part of the language? In the Scala world, there's the akka library to provide actors (but of course you could use a totally different actor system instead). Alternatively in Elixir, actors (Processes) with their send/receive functionality is built into the language and each module can essentially be an actor.

I guess the trade off is that if it's a part of the language, you get it standard and built in as a language construct. But if it's in an external library, you can use different actor systems and benchmark them accordingly.