Acting isn't exactly a particularly encouraged profession amongst the Asian-American population in my experience. Out of the dozen or so shows I've done, I've encountered 2 east-Asian actors, including at the auditions. If they're casting for the best actors without going out of their way to find Asians specifically, then it really isn't surprising to have a cast with so few Asians in it.
Also, as far as the English language in the future being unrecognizable, there is some truth to it, but considering how much media we have these days, we get as much of our language from movies, podcasts, radio, tv, etc. as we do from our environment. Language isn't going to evolve as fast because that media remains something that links all of us and remains long after it was made. Firefly went a lot further than most productions do, and I expect the reality won't be much more jarring than their guess.