Its because England had colonised most of the countries in the world plus America which has the most influence on the world through media,movies and music is English as well. I don't think another language will ever dethrone English. alot of countries teach English as a second language. so for English to be overtaken alot of countries would have to change there official language to the language challenging English plus other countries would have to start teaching this language in schools as well. For someone going to japan if they speak Spanish and no English they will suffer but if they speak English they can get around easier as alot of thing are translated there to English