Covenant languages

Translation
To facilitate communication across this vast tangled web of languages and dialects, the Covenant has fairly advanced auto-translator technology. Perhaps counterintuitively, however, personal flesh-and-blood interpreters are still favored among those of high rank and status. Where translation disks are mundane everyday objects available to most citizens of moderate wealth, interpreters are both a status symbol and a statement, especially when interacting with those they perceive to be "below" oneself. Because of their natural linguistic skills, it is often the Unggoy who are the most capable and effective interpreters, as long as they are properly educated, and many Sangheili aristocrats' (often fairly sizable) retinues include a trusted Unggoy interpreter. These interpreters are highly valued, and are often accorded honors and privileges most Unggoy can only dream of.

Liturgical Tongue
Flowery, abstruse, and lacking the versatile foundations and laconic dimensions of Sangheili grammar. Yet it is inarguably effective in the purposes it's used for. The finer points of Covenant theology can only be expressed in the Liturgical Tongue, for they are too esoteric and complex to be adequately discussed in more "mundane" languages. When you get to the nature of godhood, transcendence or the divine harmonics of the Great Journey, most languages just run out of words and ways to express e.g. locality, dimensions and time; the Liturgical Tongue has impressive ways to discuss phenomena in "no-time", in more than 3+1 dimensions, or in acausal relation to one another, for example.

Used mostly by priests.

Sailor's Tongue
I think the common trade language, or Sailors' Tongue, across the Covenant might be a creole based on Common Sangheili, but with heavier foreign influences from the "lesser" species' languages than usual, and even some words, grammatical constructions and expressions whose origins cannot be definitely pinpointed to any of the Covenant species but presumably originate from the outside. Most of the alien influence would come from Kig-Yar and some of the regional dialects of more isolated Covenant territories. Naturally, there are several versions and little central regulation of the Sailors' Tongue, and a dialect spoken in one end of the Holy Ecumene may not even be mutually intelligible with one spoken in another. Many traditionalist Sangheili aristocrats scorn the Sailors' Tongue for its purported "debasement" of the "noble" Sangheili language.