The historical Julius Caesar never said "Et tu, Brute?" Those words are famous from Shakespeare's play Julius Caesar. (Although Shakespeare didn't invent this phrase. He likely did think they were Caesar's actual last words.)

Although there were conflicting reports that Caesar did say something similar, but he said "young man" instead of "Brutus" and he said it in Greek, not Latin.

I think part of the reason everyone today thinks it's a real quote is that Shakespeare added this Latin phrase to a play that is written in English.

Meanwhile, the actual Julius Caesar, who we expect to speak Latin, is reported to have said his dying words in Greek.