St. Cyril lived from around 376 to 444 AD. The Sahidic version of the NT was translated at the end of the 2nd century. Perhaps Cyril completed the translation of the OT.
A long time ago, I used to live near a library that contained an English translation of the Coptic NT. Too bad I don't have access to this any longer.
I should have specified that he completed translation into Bohairic Coptic, which was the dialect of Upper Egypt, and which is the version still used in Coptic Orthodox liturgy, not to mention the liturgy itself, which is actually in a fifth century context more important as most Coptic speakers, especially Bohairic speakers, were illiterate, and without someone to read it to them, which happened in the Church, more precisely, in the liturgy, it was inaccessible. To this day the Coptic Church reads more of the New Testament at each Divine Liturgy than any other church, and the entire Old Testament is read during Psalmody (a component of their unusually arranged divine office that kind of corresponds to Vespers, Nocturns and Matins, although there are also the Evening Raising of Incense and Morning Raising of Incense, which typically precede Evening Psalmody and the Divine Liturgy, and the Hours of the Agpeya, which begin with the First Hour, which is obviously Prime, but the Eleventh Hour is often considered to be Vesperal, the Twelfth Hour corresponds with when Vespers happens in Greek tradition I believe (well technically it historically happened at sundown, which in a location where the day was twelve hours long would be callibrated to be eleven hours from the first hour), and then there is The Prayer of the Veil, said by clergy and monastics before bed, and so, conceptually, Vespers could be the Evening Raising of Incense, the Twelfth Hour, the Eleventh Hour, or the Evening Psalmody; historically monks used the Agpeya, which contains the Psalter and a fixed Gospel, and certain prayers for each hour, and monks tended to memorize it.
The Psalmody is also of monastic origin but developed contemporaneously with Vespers and Matins, whereas Robert Taft SJ of blessed memory, who aside from being the only Jesuit of recent years I really like, is also the greatest historian of the Divine Office in churches Eastern and Western, and who shared my view that the health of a church can be partially determined by how many people attend the Divine Office in addition to the Sunday Eucharist, in which case the Coptic Church is one of the healthiest (it really is if you look at metrics like Sunday attendance, population growth, age distribution, and other factors; their old rivals the Assyrians are also very healthy), argued that the Morning and Evening Raising of Incense were relics of a Cathedral Office. This sounds complicated, and it is, but Coptic worship, like Byzantine Rite worship, is ornate and beautiful.
Also, speaking of reading the entire Bible in one year, the Coptic Orthodox Church is the only ancient church that does that, including liturgically reading all of Revelations; the Apocalpyse, as I prefer to call it, is read in its entirety in the liturgy of Holy Saturday (Easter Even, which the Coptic Orthodox call Bright Saturday; the Eastern Orthodox call the first Saturday after Pascha Bright Sunday, and the week after Pascha Bright Week, speaking of which, on Mount Athos, many of the monasteries read on the afternoon of Holy Saturday the Apocalypse, but thus is considered to be extra-liturgical, but is certainly historically related to the Coptic practice.
Moving back to St. Cyril, the oldest of the Coptic Orthodox liturgies is a translation of what is commonly called the Divine Liturgy of St. Mark in Greek texts, which was translated during the reign of St. Cyril into Sahidic (and Bohairic) Coptic and is named for him. Most worship was in Sahidic Coptic initially, until the rise of Islam, when Bohairic became prevalent.