regular writing with a special font and perhaps all lowercase sentences
yes, that's what it has right now.
I'd maybe look at how much vocabulary Coco (the Guerrilla who was taught to speak sign-language) had
from what i recall of that and work done with symbolic languages with both chimps and birds, sentence complexity is not really high - perhaps 6 symbols max, most are on the order of caveman-speak. "coco - want - apple - question?" that sort of thing. vocabulary size wasn't bad though in the hundreds for coco i think. it would be a place to start.
if you apply it consistently
this might be the big issue right here. all existing text (or at least dialog) in the game would have to be changed to caveman speak to apply it consistently.