Latin is an ancient Indo-European language originally spoken in Latium, the region surrounding Rome. It gained wide currency, especially in Europe, as the formal language of the Roman Republic and Roman Empire and, after Rome's conversion to the Christian church, (and later to Catholicism after the "great schism") of the Roman Catholic Church. Principally through the influence of the Church, it became the language of later medieval European scholars and philosophers. It widely influenced a number of languages that came after it, including some used by the modern UNSC, such as Spanish and English. By 26th century, Latin had become extinct from every-day usage.
Onyx Sentinels were capable of attenuating their communication systems to produce COM signals on UNSC devices and use Latin phrases. After one of the Sentinels approached Ash-G099, informing him of the status of the "Shield" and demanding the "Reclaimer" to provide a proper counter-response, it referred to Ash's confused reply as a "non sequitur" (Latin for "it does not follow"). In addition, they are capable of modifying their lexicon based on samples of human language and speaking fluent English.
Some UNSC ships follow the tradition of older Earth vessels by putting Latin phrases on their ship emblems. The UNSC Marine Corps carries the tradition of the ancient United States Marine Corps by using the motto "Semper Fidelis", which is Latin for "Always Faithful".  The Office of Naval Intelligence also uses a variant of this term for their own emblem, "Semper Vigilans" ("Always Vigilant").