Of course it was preached already to the whole world whenever the Jewish and Christaian were found . We interpret the Bible with the Bible.
Acts 2.5 : men from every nation under heaven were present which again has a relative meaning . Every nation where the scatterred nation of Israel was found.
Roman Empire was the whole world to people living back then in that empire- Luke 1.2
Because the understanding of the phrase is dependent on who is interpreting these words only true worshippers can recognize the situation is ripe for the end of the world to take place,; while others may wait untill all the people living on earth will hear the Good News and then they will know the end of the world is going to take place any day or so !
But note also there are various interpretations of the end of the world. The end of the world the way I interpret can be in stages .
So there will be for sure the time after Christ returns that all living people on earth then will hear the Good News .It depends also what kind of the end we have in mind and who will be living at that time when the given end of will take place.