: So I don't (and probably never will) own an XBO, so I watched all of the
: cutscenes here . Which let me to a few questions: 1. Master Chief can't
: understand Regret's hologram without Cortana translating it. So how can he
: understand Mercy?

We know now that the primary language of the Covenant is Sangheili, so it's possible the sermon was being broadcast in Sangheili while Regret actually spoke to Chief in English.

Or it could be translation software, as Loftus suggested, although I doubt Chief's suit would only have been updated at that point, given how much intelligence ONI had on Covenant communications prior to 2552.