It was in Latin. Plutarch in his Life of Julius Caesar says he wrote it to his friend Amantius, and makes a comment about how his Greek translation of the phrase doesn’t convey the eloquence and brevity of the Latin. Suetonius reports that the phrase was used as an inscription in his triumph, in which case we can assume it was in Latin, which was used for all such formal occasions.
Plutarch reports “Let the die be cast” was in Greek, which makes sense as Caesar was quoting a play by Menander.
As for his last words, if he spoke them, Suetonius says they were in Greek as Faldage says.