English is a language widely spoken on the planet Earth. Modern English was spread around Earth as the United Kingdom expanded its borders and colonies. As a result, the language is widely spoken in such places as the United States of America, Canada, Australia and New Zealand.
The inhabitants of planet 892-IV were noted for speaking in colloquial 20th century English. (TOS: "Bread and Circuses")
Individuals that were submitted to the Sanctuary Districts were given questionnaires and told to answer all of the questions to the best of their ability. They were told that if they could not speak English an interpreter would be provided, and if they could not read, the questions would be given to them verbally. (DS9: "Past Tense, Part I")
When T'Pol was first introduced to Hoshi Sato in 2151, Hoshi spoke to T'Pol in the Vulcan language. Despite Hoshi's good intentions, T'Pol replied to her coldly, stating "I was instructed to speak English on this voyage and I'd appreciate it if you'd respect that." (ENT: "Broken Bow")
In early 2152, upon taking Vulcan ambassador V'Lar aboard the Earth starship Enterprise, T'Pol addressed V'Lar in the Vulcan language. However, V'Lar insisted that they not speak in the Vulcan tongue, as she had been anticipating the opportunity to practice her English. (ENT: "Fallen Hero")
Later yet that year, Jonathan Archer encountered a drayjin that appeared to him in the form of a Human female. This female spoke to Archer, much to his surprise, as his initial reply to her was a statement of the obvious: "You're speaking English." (ENT: "Rogue Planet")
When Enterprise encountered an automated repair station, Captain Archer noted that all of the panels were in English. T'Pol stated the computer must have scanned their database. (ENT: "Dead Stop")
When the USS Enterprise encountered a Melkotian buoy, it spoke to the crew in their native languages, which included English, as heard by James T. Kirk. (TOS: "Spectre of the Gun")
When the crew of the USS Enterprise-D pulled up the Treaty of Armens, it included both the English language version alongside the Sheliak language version. (TNG: "The Ensigns of Command")