In which areas of the world is English most useless?
The EF English Proficiency Index answers this question very directly. Here is a map of the results that were published in 2015:
Previous answers about Eastern Europe are accurate in my experience. From traveling for the past 14 months through 18 countries on 4 continents, I’d add a few extra problematic countries for travelers who only know English:
China – Of course there are loads of people who speak English in China, but good luck finding them. My wife and I travelled there for 2 months and I don’t think we ever found somebody who spoke English when we were looking for them. They would come and find us sometimes though. I guess the best bet would be just to look especially lost and wait for somebody to come to you.
Sudan – In Khartoum many of the well-dressed young people speak very good English, as well as pretty much all waiters (some of them don’t even speak Arabic), but, as soon as you get away from the capital almost nobody knows English, including the police.
Ethiopia – Just don’t expect anybody on the street to know any more English than “you, you, you”, which they always seem to love to yell at foreigners, and the occasional “give me money”.
Myanmar – Pretty much the same as Ethiopia. Almost no English anywhere.
Brazil – I speak Portuguese, so it’s not an issue for me, but my wife doesn’t, so it has become quite clear that a very low percentage of people speak English well in Brazil.