Board Thread:Destiny Forum/@comment-5650583-20131111135917/@comment-5904951-20131208145227

The Fallen likely speak English due to the reason that the Covenant Empire learned to. It is strategically advantageous to understand the language of your enemies. Applications of this skill are intercepting and translating enemy transmissions, understanding battlefield directives shouted by the enemy, interrogation, and simple communication.

However while it is true that the many species of the Covenant learned to speak English by 2552, they could do so throughout the majority of the war as well. Halo 2's frequent English usage was for gameplay purposes only, spoken English generally reserved for interspecies communication.

Though going back to the Fallen, Rixis' application of his English was obviously to convey his malicious intent to the Guardian intruders, more than likely to intimidate them.