Language is spoken before it is written. The folks who were the first to speak the Welsh language is presented by Bede in his book "The Ecclesiastical History of The English Nation"written between 680 - 703 AD. In chapter I, he writes "This island at present....contains five nations, the English [Anglo-Saxon], Britons [Brythonic], Scots [Gaelic], Picts [Pictish], and Latins [Latin], each in its own peculiar dialect..." He continues to write "The Latin tongue is, by the study of the Scriptures, become common to all the rest." So by 700 AD Latin was the chief language of the clerks, and writers of the time, but had also become a language between the nations. [Please note that the brackets are mine and not Bede's.]
Now for the Celts, the spoken language had become the only means to transmit their legends and stories. To accomplish this generation after generation, a special position was established to manage this endeavor. The Bard he was called.
Bede's text is taken from: Everyman's Library 479, History, J.M. Dent & Sons LTD, 1954, London, p. 5.
The drawing is taken from : Wales Her Origins Struggles And Later History Institutions and Manners, by Gilbert Stone, Frederick A. Stokes Co., NY, 1915. [Please note that the drawing is by Edward Jones, 1784...those Jones are everywhere.]