I think this old thread should just about answer your question. Probably, though, Suetonius' version (καὶ σὺ τέκνον), which he only reports as something that other people say, were not Caesar's last words. Despite the hearsay that Suetonius reports both he and Plutarch state as fact that Caesar did not speak after the first blow was struck. More likely his last words were his response to Casca's initial attack, which Suetonius reports were "ista quidem vis est" ("Truly this is violence!") but which Plutarch says were (if you'll excuse my rather loose translation) "μιαρώτατε Κάσκα, τί ποιεῖς;": "Casca you bastard, what are you doing?!" Plutarch says that Caesar said this in Latin...but he doesn't actually report the Latin, he gives the Greek. After that first outburst, though, both Suetonius and Plutarch are in agreement that Caesar said nothing.