The King's Speech is the common language of Calibran, spoken more or less by every race, and used almost exclusively at the court of the King. It has also, to some extent, become the language of scholarship and learning. While the noble elf language still is considered the best for expressing ideas and preserving knowledge, it is so difficult that few other races learn to speak it even a little. The King's Speech has become more popular as a result.
The language itself has its roots in a trade language that was used by merchants and sailors during the Dragon Wars, and the years prior to those conflicts. When Aramnor ascended the Dragon Throne, it became apparent that communication would be one of the key obstacles to creating a truly unified kingdom. He set a group of elven and human scholars, and minotaur shamans and wise men, to creating a tongue that could become a common language, and start to bind the kingdom together.
This group of scholars used the trade language as a basis for creating a more complete and whole language. Aramnor helped to popularize it by making it the official language of his court, so that those seeking office or favor there must learn to speak it.
Over the centuries it has evolved and been added to, becoming a living language that is now spoken by more people than any other.