The Language of the Future
TL;DR: The reason that English is spoken in your fantasy world is because a powerful mage traveled to Earth, thought it was the language of the future, and introduced English as the official government language. After generations of use, eventually everyone in the nation spoke English.
Dear <insert native name here>,
My explorations of yours and nearby nations have been fascinating. Seeing Ewoks play with the sapling Ents was particularly cute. But one question has been incessantly nagging at me. As you well know, I come from a extremely distant nation. You would not be able to comprehend how far it is away from your nation. At the place that I come from, people speak several different languages, depending in which nations they live. I have also noted the same thing to be true here. While most languages that I have heard here have nothing in common with the languages spoken at home, the language that is predominant in your particular nation is not only similar, but virtually identical to my native language. Our two cultures are so vastly different, along with the distances between our nations, that I cannot comprehend why our languages are identical. Can you tell me what you you know about English and how it came to be spoken in your nation?
<insert hero name here>
Dear <insert hero name here>,
I too am intensely curious as to how our languages became so similar. Fortunately, being a royal librarian gives me access to many of our nations most ancient documents. To sate mine and your curiosity, I searched these documents and found much knowledge that has long been forgotten by our general population.
Many and many generations ago, our nation was newly formed from the ranks of several races, several of which you have already met. Each race had their own language, and it was necessary for us to choose a national common language for legal purposes. To resolve the quandary, a great council was called of the wisest elders of all our races.
At the meeting, these wise elders could not settle on any one language. Whenever one language was proposed, the representatives of groups that didn't speak that language would argue that selecting that language would provide an unfair advantage to the races that already spoke the language. After months of this constant arguing, it seemed that all hope was lost.
One day, the great and powerful mage Yaj rose from his seat. The entire room was suddenly silent. Yaj had single-handedly destroyed more of the invading forces that brought the nation together than all other forces combined. He then announced to the congregation, "I have found a language. In the far future, a future so distant that oceans have moved to where mountains are and mountains where oceans, most of the inhabitants of the world speaks one language. This language is vastly different from any of our current languages. Since it is the language of the future, I propose that our own nation adopts this language." He then sat down.
Our Elders quickly decided to adopt Yaj's solution, though it was more because of their fear of disagreeing with him than the reason-ability of the idea. After their agreement, he produced several strange square manuscripts written in English. He then performed magic to make the manuscripts never fade or wear. He also performed spells of universal understanding on these books so that anyone would be able to learn English. Some of these manuscripts provided strict rules for English. Others had long lists of words that we could speak. Interestingly, the manuscripts even had a name for strange square manuscripts. They were called books. The most common type of manuscript merely showed examples of that language. After that point, our nation slowly adopted the new language over the course of generations.
Maybe your nation conquers the world in the far future. That is really the only explanation that explains why you have the same language as us. If you find more information that helps explains this quandary, I would be glad to hear of it.
<insert native name here>
After receiving this letter, your hero comes to the conclusion that Yaj had been wrong when he thought he had traveled to the future of the fantasy world. He must of instead traveled to a fairly modern Earth to a place that speaks English like America or England, and then assumed that it was the future. The square manuscripts were obviously books written on Earth but preserved on this world using powerful magic.
Note: Inspired by the explanation that the Divide Trilogy uses to explain a planet-wide English speaking fantasy population.