Mrs. Neal, the sole American character in the film, only speaks English, yet she has no trouble understanding the others, who all speak Japanese. If she did know the language, she would have used it, even haltingly, or at least asked someone to translate (obviously, this does not apply to the English-dubbed version).
In the film's first shot of the red sky from the airplane's cockpit, before the UFO appears for the first time, the bottom border of the red-hued transparent plastic covering causing the effect is visible, resulting in a long sliver of normal blue sky below it.