You said "most" and not all. So the best was to reach most people would be to do what tech or media companies do. Create a viral campaign and entice news media with click-baity headlines so that they get more views and make money. You will have to translate the "Hello World" sentence into local languages for maximum reach.
Also, you did not say to do this cheaply. So for people with no internet, I would fly air balloons with massive "Hello World" banners.
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You said "most" and not all. So the best was to reach most people would be to do what tech or media companies do. Create a viral campaign and entice news media with click-baity headlines so that they get more views and make money. You will have to translate the "Hello World" sentence into local languages for maximum reach.
Also, you did not say to do this cheaply. So for people with no internet, I would fly air balloons with massive "Hello World" banners.