"Klaatu barada nikto": as Wikipedia says, "no translation of the phrase was stated in the film". They go on: "Philosophy professor Aeon J. Skoble speculates the famous phrase is a safe-word that is part of a fail-safe feature used during the diplomatic missions such as the one Klaatu and Gort make to Earth. With the use of the safe-word, Gort's deadly force can be deactivated in the event the robot is mistakenly triggered into a defensive posture ... the phrase apparently tells Gort that Klaatu considers escalation unnecessary.'
Well, what might we say? My first thought was that 'nikto' (that 'k') must have something to do with the Greek for victory (let's say, 'Klaatu casts victory into the abyss'; what a mournful gloss on the film that would provide...) But then I thought: no. Clearly this is Latin we're dealing with, not Greek. What else would superadvanced alien robots speak? So: tuba means, as it still does, 'a trumpet, a horn'; rada (or ræda) means 'a travelling carriage, a cart with four wheels'; and nicto means 'I blink' or 'I wink' ('to move the eyelids up and down, to wink, to blink'). In other words, the phrase is saying (starting with an obvious friendly diminutive): 'Klaa: the blare of these car-horns makes me blink'.
Makes sense, in context, I think.