Chapter 17: A Sluggish Memory points to this however:
That muggle what my sister took a fancy to, that Muggle what lives in the big house over the way.' said Morfin, and he spat unexpectedly upon the floor between them.
Later..
"Voldemort Stupefied his uncle, took his wand, and proceeded across the valley to 'the big house over the way'"
Looks to me as if he's repeating what Morfin said in parseltongue verbatim. Which says to me that he understands it (which isn't strange at all, since he also speaks Mermish.
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See You In Another Life, Brother.
New York Giants - The Patriot Slayers