From Halopedia, the Halo wiki
The language of the San'Shyuum is similar to Medieval Gregorian-style chanting, based on religious sermons by the High Prophet of Regret on Installation 05. Whether this is spoken by all San'Shyuum, or if it is a special dialect for religious purposes is unknown. One of the terms of the Writ of Union was for the San'Shyuum to adopt the Sangheili language as their own, an act viewed by the Sangheili as the single greatest concession the San'Shyuum made during the treaty process that effectively established their own culture as the foundation of the nascent Covenant Empire. However, to the San'Shyuum, this was a non-issue and they were willing to trade their language for power.
The San'Shyuum also used non-verbal communication, in the form of elaborate or simple hand gestures, though some involved movement of the head. Most other races, such as the Sangheili, were completely unaware of this. The San'Shyuum were also able to convey expressions through these gestures, such as satisfaction, irony, obeisance, amusement, respect, and empathy. An extra flip of the thumb was used to show a gesture was being used mockingly. Accents could also be conveyed through these gestures. The Stoics on Janjur Qom seemed to have used the same hand gestures.
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