[quote=Rhombic]
The player would represent (roleplay as a) tribe/community/town/group of people/llendwnyraeddIt is not supposed to be Welsh, but a related language. Sort of those spoken in Britain before the Roman conquest.
[/quote]

[url=http://en.wikipedia.org/wiki/Common_Brittonic]Common Brittonic[/url] is what you are looking for.