One criterion is the number of native speakers that a language happens to have. A second is the extent to which a language is geographically dispersed: in how many continents and countries is it used or is a knowledge of it necessary? A third is "vhicular load": to what extent is it a medium for science and literature or any other highly regarded cultural manifestation - including "way of life"? A fourth is the economic and political influence of those who speak it as "their own" language.
The reasons for the demand of English have surely become clear. To put it bluntly, English is a top requirement for those seeking jobs - and is often the language in which most of the business of "good jobs" is conducted. One needs it for access to at least one half of the world's scientific literature. It is thus intimately associated with technological aid. Not only is ti the univesal language of international aviation, shipping and sport: it is to considerable degree of universal language of literacy and public communication. Seigfried Muller (Formes Director of the Languages-of-the-World Arquives in the US Department of Education) has estimated that about 60 per cent of world's radio broadcasts and 70 per cent of the world's mail are in English. The great manufacturing countries Germany and Japan use English as their principal advertising ans sales medium; it is the language of automation and computer technolgy.
A Grammar of Contemporary English