|There is more information available on this subject at English on the English Wikipedia.|
English is a West Germanic language originating in England, and the first language for most people in Australia, Canada, the Commonwealth Caribbean, Ireland, New Zealand, the United Kingdom and the United States of America (also commonly known as the Anglosphere).
In the Halo Universe
Covenant language translation software intercepted human COM transmissions after The Battle of Harvest, and thus the Covenant learned English quickly. From 2525 to 2552, the Covenant used this knowledge mainly to broadcast florid curses at Human colonies and ships before destroying them but never actually held conversation with Humans.
UNSC translation software has been able to decrypt a number of Covenant languages, most notably those of the Grunts, Elites, and Prophets. By the time of the Battle of Earth, this software could automatically interpret everything the other races said, and it has apparently been installed in the equipment of all UNSC military personnel, as all personnel appear to have the ability to understand what members of these races are saying. This is the reason that Grunts, Elites, Brutes, and Prophets heard in Halo 2 seem to be speaking English. Some of the members of the Covenant know how to speak English, mainly Elites and Grunts. In Halo: Reach however the Covenant cannot speak English.