Variety is one of the most beautiful things in life. If everyone spoke the same language (English), the world would lose its intrigue. The tourism industries of many countries and subsequently jobs may suffer. Many people and experts predict that by the year 2100, half of the languages spoken today will be lost. Subsequently people will become more Westernized and their sense of cultural identity will be lost.
I can visibly see this taking effect upon people in my home country. Amongst the modern generation of youth, many natives are beginning to speak a combination of English and their mother tongue and idolize Western culture and celebrity lifestyles. Moreover they are beginning to lose interest in their cultural teachings. If everyone spoke English, many valuable teachings would become part of a bygone era. For example many traditional African cultures teach that people should never ever show any form of disrespect towards towards elders. Today it is not very rare anymore to see some of these people showing disrespect to elders.
People would all begin to all gain Western values and ideals and those taught by other cultures may begin to cease.
Although I think that everybody in the world would never speak only English, due to the complex and possibly controversial procedures which may be required, I do think that it would bear some positives. For instance, it would decrease conflict between people due to misunderstandings. People would begin to become more unified and feel a sense of togetherness. Countries and people would also save vast amounts of money by avoiding modifications and processing to documents and education would become cheaper and more accessible to all as everyone could use the same material.