It's just like in science fiction movies... where they travel to the vast reaches of space... and come across an alien race... and you've got it... they speak English!
Unless it's like Star Trek and there is a universal translator.
But my guess is that all "speaking" races on Oddworld would speak English for this reason.
It'd be hard to use Gamespeak if it were in an Odd language!
Abe Babe...