Old English developed from a set of Anglo-Frisian or North Sea Germanic dialects originally spoken by Germanic tribes traditionally known as the Angles, Saxons, and Jutes. As the Anglo-Saxons became dominant in England, their language replaced the languages of Roman Britain: Common Brittonic, a Celtic language, and Latin, brought to Britain by Roman invasion. Old English had four main dialects, associated with particular Anglo-Saxon kingdoms: Mercian, Northumbrian, Kentish and West Saxon. It was West Saxon that formed the basis for the literary standard of the later Old English period, although the dominant forms of Middle and Modern English would develop mainly from Mercian. The speech of eastern and northern parts of England was subject to strong Old Norse influence due to Scandinavian rule and settlement beginning in the 9th century.
Try to answer this question if possible (otherwise reply "unanswerable"): Whose language was replaced by Common Brittonic?
unanswerable