I saw this interesting question on the website Quora, and I decided to answer the question myself. Here is my take.
It is not as simple as what many people imagine. Let’s start by assuming that the power transition from the US to China does happen, then Mandarin Chinese, let alone 400+ other languages from China, will not automatically be the world’s new lingua franca.
To begin with, one needs to understand why English becomes the world’s de facto lingua franca. Consider the history of the British Empire. “The sun never sets in the British Empire,” so they say, an Empire that once conquered one-sixth of the whole world. It was clearly not the United States that caused English to be the world’s main language.
That wide sphere of influence causes English to be widely adopted, indeed. In all these colonies, English is certainly the official language, at least for government and official matters. While there are variations and local languages, English prevails anyway.
I believe (I don’t claim to be a linguist or philologist myself) that there is another important thing that happens: the fact that these English speakers are willing to learn the local languages, therefore allowing intercultural and interlingual interaction to happen. Willingness to interact constructively with other language is therefore important.
To China, not so much. Data says that more than 400 million Chinese are learning English every year. Yet, during a visit to China a few months ago, to the third largest city in China, barely anyone speaks English—not even in those restaurants that are considered to be American! Talk about spreading influence, I don’t think China will be able to make its interests across unless it opens itself to elements outside of its own culture.
Of course, today, learning Chinese is very much encouraged, and considered to be among the world’s most important language for the nearest future. Yet even if China does become superpower (the characteristics and signals of which we cannot know for sure), it is by no means automatic.