Dead languages?

Orthodox are used to contrasting the Latin Church's traditional imposition of the Latin language on the liturgical usage of all Western Catholics before the 1960s, with our supposed tradition of worship in the vernacular. But many of us worship at least in part in "dead languages" of our own, specifically Koine Greek (that of the Bible, more or less), Church Slavonic, and IIUC a form of Arabic no longer spoken. Who rises to shed light on this but Her Majesty's Canadian Ministry of National Defence (sic)?! It's on a page intended to inform Canadian Forces officers about potential religious requirements of their charges (but not in all ways accurate on Orthodoxy):

The Orthodox Churches have historically been committed to the use of the vernacular in the liturgy, whereas the Roman Catholic Church used Latin until modern times. However, the conservative nature of the Orthodox Churches has resulted in the retention of old spoken forms of national languages in the liturgy. For example, the Greek churches today use the ancient Greek of Byzantium in the liturgy, and some Slavic churches use 17th-century forms of Slavic languages. This is somewhat analogous to the use of 16th-century English in the King James version of the Bible and in The Book of Common Prayer of the Anglicans.

(Strangely, the site also incorporates information on groups most officers probably won't encounter in their units: Quakers, Hutterites, Jehovah's Witnesses, and Doukhobors! Gotta love Canadian inclusiveness!)