Edit: Were you being sarcastic? I don't think it's exactly the same, because being able to compile many languages down to a single language is easier than also having to go in the other direction.
Original comment: Yeah, that would be one solution, but I think the shared VM helps so you can easily access libraries written in different languages. I remember why (of Ruby fame) trying to run Ruby code on Google App Engine back when it was Python-only. He discovered that Ruby bytecode is nearly identical to Python bytecode. Like me, he didn't understand why Python and Ruby still need to be on separate runtimes.
No, I wasn't being sarcastic. If you want to use libraries written in Python in Ruby (just as an example), you need to be able to convert all the Pythonisms to Rubyisms and/or Rubyisms to Pythonisms. That is the case whether you are doing source-to-source rewriting or using a shared VM.
Note that the JVM and .NET runtimes do pretty much exactly what was asked--they run programs written in multiple languages on the same VM. Because the "native" VMs for Python and Ruby are so bad, the JVM and .NET VMs can even out-perform the native ones for most tasks.
Original comment: Yeah, that would be one solution, but I think the shared VM helps so you can easily access libraries written in different languages. I remember why (of Ruby fame) trying to run Ruby code on Google App Engine back when it was Python-only. He discovered that Ruby bytecode is nearly identical to Python bytecode. Like me, he didn't understand why Python and Ruby still need to be on separate runtimes.