When has that been clear? Once the change is complete, there is no evidence that any trace of the former human remains...
I agree that we have no clarity here. I think the human is "gone" (soul moves on to "What Comes After"), but
at least one element remains: the Rampire still speaks whatever native language the human spoke.
That could be nothing more than Doylist need: "can talk, and understand, so Harry can snark at them" may be the whole of the matter!
I have 2 working Watsonian theories, though ...
First, that the nascent vampire (within the half-turned but still human host) might be learning: learning languages, learning to recognize friends and family, co-workers, learning habits and patterns and behavioral quirks and etc. This gives them a
huge leg up, when they finally turn, as they can fake being still-human in their original lives as well as more-generally.
Second, that maybe the vampire binds the ghost of the newly-dead, uses their knowledge & the binding to emulate the human, and "learns by doing" until, eventually, the ghost fades away.
I slightly-prefer the first one... it helps to explain much of the difference between the feral "blood slave' vampires and those more-in-control: those who lived long as half-rampires are simply much more well-adjusted to acting human, fitting-in within human societies, etc.
