...how can you explain the fact that if a Buddha still speaks to sentient beings, she must apprehend the concepts which those sentient beings communicate?
Those concepts are necessarily conventional.
So, does it not follow, that a Buddha must be able to apprehend the conventional phenomena of ordinary speech?
First, I don't regard that as a fact. The rest of your statement depends upon me accepting that fact, but I don't.
The primary Mahayāna sutra metaphor for a Buddha is a wishfulfilling gem because a wishfulfilling automatically gem fulfills the wishes of sentient beings without concepts.