It could run JavaScript, but this is a 16-bit processor we're talking about here, with minimal RAM. It's more likely we'll be using C and BASIC ;)
If the current level of interest persists, by the time the game launches, I imagine that the vast majority of people will be downloading and running programs written by others. These will have been pored over and optimized to an extent that most of us would be unable to achieve by ourselves, and it will not pay to roll your own trivial implementation. I'd be curious to see what Notch can do to still encourage people to learn how the CPU works themselves. If the environment and game dynamics are rich enough, perhaps this will not really be a problem?
Have some ship customisation stuff tied to assembly programming. People love changing the colour of carpets or wearing hats or collecting and displaying fossils.
Then allow the community to create simple how to guides - "This program will do $THIS_THING; here's how it works; now try to change it to do something slightly different."
I guess Codecademy should have a DCPU section too.
Agreed that higher-level langs would be helpful for "newbies". I wonder if this was at all inspire by Schemaerse (google it, but TL;DR it's a space game written mostly with Postgres triggers where /you build the game client/ in whatever you want to use to interface with the pgsql :)
The Haskell embedding is very likely to head in that direction.
(See e.g. in this style : http://www.fftw.org/faq/section4.html#whyfast or this style : http://www.cse.unsw.edu.au/~chak/papers/polymer.pdf -- code generation + DSL + constraint solver for instruction level timings).
At least, that's what I'd do.
[0] http://en.wikipedia.org/wiki/Game_Oriented_Assembly_Lisp
The task builder.
TKB allowed (much) larger applications to run in the address space of a 16-bit DEC PDP-11, using what were called overlays, and overlay trees.
With overlays, the application call tree within an application was analyzed and implemented to allow various sections of code within a tree of subroutine calls to be paged out to backing storage.
If the underlying "processor" is fast enough and if you have enough swap space available, then you can stuff a whole lot of code into a 16-bit address space. Just not all loaded in physical memory at once.
And debugging it... Shudder...
It was more similar to vi + integrated compiler as an IDE.