A vernacular language is a regional language, as distinct from an international literary language, such as Latin and Greek. Throughout the Middle Ages certain texts, notably those of a liturgical character, were generally in Latin (although biblical texts were gradually translated into the vernacular). The development of Western vernacular literacy began at least as early as the sixth century in Ireland and Celtic Britain and spread to England in the following century. Spain and Frankia followed suit later. The growth of secular literacy beginning in the twelfth century stimulated an increased use of the vernacular in texts. See also Bible.
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).