Bohairic is the dialect of the Nile Delta, and it displaced Sahidic as the literary standard of Coptic from around the eleventh century, by which time the patriarchate had moved north. It is the dialect in which the Coptic Orthodox Church still conducts its liturgy, so Bohairic is the one form of the Egyptian language that has never stopped being used out loud, in an unbroken line running back through Demotic and hieroglyphic Egyptian to the third millennium BCE.
O. H. E. Burmester's detailed description of the liturgical services sets out where the language actually stands within a service. The three anaphoras in ordinary use, of Saint Basil, of Saint Gregory the Theologian and of Saint Cyril, are Bohairic, as are the hymns of the Coptic psalmody and the responses the congregation sings, while the readings and much of the preaching are in Arabic, which has been the spoken language of Egyptian Christians for a thousand years.
The result is a service conducted in two languages at once, with congregations following Bohairic that they may not otherwise speak, supported by printed texts that set the Coptic and the Arabic side by side. The pronunciation of Coptic itself became a controversy in the nineteenth century, when a reformed system based on modern Greek values was introduced and largely replaced the older Egyptian pronunciation, and both are still defended by people who use them.