It's hard to come up with a definitive answer for this question, since there are various factors that play a part in this.

 

First of all, it'd probably be very hard to agree on a language. There are about 5000 languages in the world and language is usually closely intertwined with things such as national pride. As you can imagine, a great many countries would like to see their language as the "universal language" in the world.
Even if we exclude politics, what criteria should we use to choose the world language? The one with the most native speakers? That'd probably be Mandarin with approx. 900 million native speakers. Or what about English, since it's already very widespread?

 

Nowadays, English is commonly seen as a kind of world language. Due to various reasons (English colonialism, US dominance past WWII (not just military dominance, but also culture and science)), English has spread over the world and is taught as a mandatory second language in many countries.
As Robert Phillipson claims in his 1992 book Linguistic Imperialism, private as well as government organisations spent huge amounts of money in an effort to spread the use of English during the 20th century (mainly by advertising the acquisition of Englisch as a second language in other countries and promoting British and US culture, e.g. through Hollywood movies).
In 1935, the British government founded the British Council, which was tasked with spreading British culture, arts and philisophy (as well as language). The US counterpart was the United States Information Agency (USIA), which was "responsible for [...] the spread of American English in areas of US influence" until it was dissolved in 1999. However, these claims are disputed among scholars, with some calling Phillipson a "conspiracy theorist".
Linguists who work with World Englishes (the variations of English used around the world) usually support the idea of multilingualism. They criticise the idea of learning English as a second language with the goal to "imitate the native speaker perfectly" and argue that Standart English shouldn't be the "one single base of authority, prestige and normativity".

 

As you can see, the idea of a world language (esp. Englisch as a world language) is highly disputed even among scholars. Trying to select one language as a world language would probably cause huge disputes among the governments or the world, with each trying to "win" the competition and many refusing to replace their own language with the world language.

 

Source: Mesthrie, Rajend, and Rakesh M. Bhatt. 2008. World Englishes: The Study of New Linguistic Varieties. Cambridge: Cambridge University Press.