The main difference between a lingua franca and an official language is that the former is adopted in an informal way and the latter in a formal way. For example, English is a lingua franca in many parts of the world (people who are not English-speakers use it to communicate between themselves) and is the official language in UK because it is written in the Law.
Languages were built based on culture and land. Or maybe it should be said that culture and language are intertwined and were developed based on the land where they emerged. In ancient times there was no need to formally state which the language of each land was. People spoke it since birth and that was enough. Back then, there were no countries and borders were often changing, mostly due to frequent wars that aimed to extend the territory. On the other hand, there were travellers and traders moving from place to place. These foreigners would bring novelties and new languages. In order to communicate, they would learn the language of that land, serving as translators when needed, or they would use another language that was known by both parties, a lingua franca.
And then, there came the empires. For emperors, extending the territory was not enough. They aimed to impose their culture by imposing their language. One of the most remarkable examples was the Roman Empire. The Roman Empire included most of the current European western countries, some parts of the North Africa, and some parts of the Middle East. Latin was the lingua franca in the entire empire and it became the basis of Latin languages like Italian, French, Spanish, Portuguese, and Romanian.
Later, the Age of Discovery and the subsequent colonialism took this strategy to another level. Besides extending the territory and imposing their language, Europeans sent their own people to live there. In the meantime, the Treaties of Westphalia created the concept of countries: nations that had their own territory defined by borders, their own people, their own culture, and, of course, their own language. National identity began to take shape… and, therefore, the necessity for defining an official language. So, European countries made their own language official in Europe and in their colonies around the world. That is why, for example, most countries in South America speak Spanish (they were colonies of Spain) and USA speak English (they were colonies of the UK).
When the USA became a superpower in the world, they did not aim to extend their territory or to colonise other lands. They used a new strategy: making deals. However, making their language (English) the lingua franca worldwide remained as important as it was before. Currently, it is the main language for international business, international politics, international tourism, international sports events… or just to communicate with people from other countries, regardless they come from English-speaking countries or not. Thus, learning English in school as second language is compulsory (or almost) in many countries. Even in China, learning English is important.
China has plans to take over the world using a similar strategy as the USA: making deals… and going to western countries to learn everything they know and then go back to China and make it better. However, they also understand the power of language. Therefore, it is not a coincidence that an increasing number of Confucius Institutes in the world is giving the opportunity for people to learn Chinese. In truth, there are more and more people learning Chinese for business purposes, for example. And this is how it starts…
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