Language is different from conceptual understanding. The ancients had a word for "lightning"* but they had no concept of electricity, ionization, or positive / negative charges. However, they still had a word. So having language wouldn't have enabled Adam to understand death.
* "barak" or some variant thereof, in case you were wondering
What the serpent said was a qualified truth. If we were not encumbered by moral compunctions, we could hold great power over nature. What the serpent conveniently neglected to say is that this power would be the cause of our inevitable perdition.
True; the serpent didn't say this. But God didn't either.