There is no question that English has emerged as a world language because of the British Empire. When countries in the East and Africa were conquered, the governship became English and, thus, the official language. So, anyone who wished to advance in the English colonies certainly needed to learn English. And, because England was such a world power, its language extended across continents. With the emergence of America's power and economic opportunities, the continuation of English as a world language was confirmed. Added to this, was the "Ugly American" businessman, tourist, etc. who knew no other language, so the people who wished to do business with them found a knowledge of English beneficial.
It is worthy of note, however, that English is not the international language of such organizations as the United Nations. Rather, French is employed because French is a more precise language. With English words having so many different meanings and nuances, many official documents, treaties between countries, are translated into French since individual words have more exact meanings in this language and such meanings cannot as easily be misinterpreted as in English.
English comes from German, so its roots are in Europe already. The British Isles were conquered by Germanic tribes among others for years before the Battle of Hastings in 1066 settled that it would be the French and English from that point on who would haggle for control of the islands. With the rising power of Great Britain and then later the USA, countries realized that to negotiate marriages among the royal families of Spain, Germany, Italy, Russia, Holland, etc. as well as political treaties and other such agreements, a common language would be necessary. English seems to be the one.
English is not simply becoming a world language--it is one and has been for years. For example, the language of flight is English--all pilots must be able to communicate in English. There are other examples such as this in the fields of medicine and business that prove English's reach.
While other languages are gaining a stronghold in the global community, I do not think that the tradition of so many countries and careers relying on English will pass away. A personal example of this is from my time teaching English as a foreign language in South Korea. As I learned Korean and spoke it as much as possible, South Koreans would respond with surprise, "Why are you trying to learn Korean? It is not a world language like English." Moreover, because of the extremely competitive job market in S. Korea, many of my adult students were forced to improve their English. They told me that next to which university they were able to "test into," the most important factor in obtaining a high-paying, respected job in Korea is one's grasp of the English language. They even said that it did not matter what one's university major was as much as what his/her English skills are.
As borders are being crossed in an internet driven and more cosmopolitan world, English is becoming a world language. The other posts have pointed this out very well. Yet, other languages are gaining emergence in this setting, as well. Spanish, for example, is becoming more prominent around the world. Additionally, Chinese is becoming an almost necessity as global competition increases. With the presence of greater cosmopolitanism, fluency in multiple languages is becoming a prerequisite for success.
Indeed, English is spoken in many more parts of the world than other languages. What's more, English has deviations and variations from country to country. The British, for instance (along with many other European and pseudo-European countries), insist that words like "color, favor, honor, and flavor" be spelled with the extra "u" between the final "o" and "r," resulting in words like "colour, favour, honour, and flavour," depending on where you are in the world.
Spanish has had such deviations and discrepancies for centuries, as different Hispanic populations around the world have evolved the language to best suit their needs -- for some, that means traditional Castillian Spanish is spoken, while for others, it means ghetto slang is the rule of the day.
Still, considering all this, it is rather interesting that English has "caught on" as the global language taught in schools. Certainly it is beneficial to master more than one language for many reasons, but the fact that English is looked to as having industrial and commercial importance certainly bodes well for its future, in particular.
I can't cite statistics to support it, but my perception has been that in the other industrialized nations of the world, more citizens speak English than Americans speak a foreign language. (The rapid population growth of those in the United States who can speak Spanish may impact that ratio, however.) I am always amazed by the number of people I have met who speak English quite well as a second language. I have been told by my foreign friends that learning English was required in their schools.
From a historical perspective, the British empire surely spread the English language to many corners of the world, which contributed to English becoming a world language.
enhlish as a world language
English is indeed best qualified to be called a world language. Though English is the mother tongue of a large proportion of the total population of the world, this is not the reason which makes it best qualified for the title of world language. English is, and is best suited to be, used as a world language because it is the most widely used language for communication between people of different countries.
It is quite true that people today are learning and using many foreign languages other than English. But people learn these other languages such as Chinese and Mexican to be able to communicate most effectively with people of countries where these are spoken. In contrast, English is useful for communicating with people in many different nations across the world, including in regions where English is not the language of local people.