Assuming C++ is out of the running, the alternative to Java is basically Scala. And that's it. Maybe Go will be an alternative in the eventual future, but right now it lacks the library & tool support.
There's also nothing wrong with writing your v1 in one language and then rewriting in a different one if you need to scale. You will probably need to do this anyway, even if you start in C++ or Java, because the architecture needed to scale by an order of magnitude is usually very different from the architecture needed to iterate quickly.
There's also nothing wrong with writing your v1 in one language and then rewriting in a different one if you need to scale. You will probably need to do this anyway, even if you start in C++ or Java, because the architecture needed to scale by an order of magnitude is usually very different from the architecture needed to iterate quickly.