Mathematics and computer science use created languages called formal languages (like computer programming languages), but these may or may not be 'true' languages. Mathematics itself is seen as a language by many. Some people consider musical notation to be a way of writing the musical language.
Chinese is the language with the most native speakers in the world, but Chinese is not really a language. It is a close family of dialects, some of which are as different as Romance languages are from one another.
English is often called "the international language". It is the main second language of the world and the language of science, business, and entertainment.
English as a first language: 380 million.
English as an official second language: up to 300 million.
English taught as a second language, but with no official status: anyone's guess, up to 1000 million.
Chinese (Mandarin): 390 million native speakers.
Some languages are made up so that a lot of people around the world can learn them, without the new languages being tied to any specific country or place. One of the most popular of these languages is Esperanto, which is sometimes called "La Internacia Lingvo," or "The International Language." Another of these languages is called Volapuk, which was popular about a hundred years ago but is much less popular now. It has mostly been replaced by languages like Esperanto, Interlingua, and Ido.
Part of the reason that Volapuk became unpopular is that some sounds are hard to say for people who speak Spanish or English, two of the most widely spoken languages in the world.