Hwo do we know that it is latin that the bust was speaking. It could have been German for all Harry Ron and Hermione knew! alicenadjasperforever peoples (send me an owl, no howlers or ill bite) 23:32, March 29, 2010 (UTC)

Because the narrative says so.
"'Here,' he said; he threw the Invisibility Cloak over both of them and they stood listening carefully over the Latin mumblings of the bust in front of them."
—OP chapter 32[src]
--  Seth Cooper  owl post! 23:51, March 29, 2010 (UTC)