Even though we don't see it, Amoridere stated that besidess the swear words and few sentences she can string up, the number of which being three (at the time of this writing), Reimu communicates on the level a rehabilitated feral child might. Of course, if you take into account that Reimu was age regressed to about two (at the time, it is implied that it has been about year since then) and that she was virtually denied contact besides being injected with needles and, later on, apparently brainwashed, so that might have played a part as to why she would talk like that, having lost most of what language skills she had by that point and having to relearn them at some point later, leading to this bit of Fridge Brilliance regarding the way she talks.
