The language the Druids speak in the series is Welsh, which is one of the ancient languages of Britain and is still in use throughout Wales today.
The song played during the intro of season 1 of the series is called Hurdy Gurdy Man. It was released in 1968 by singer/songwriter, Donovan Leitch. In season 2 of the series, the intro song was switched to Season of the Witch, also by Donovan Leitch.
David Morrissey also played a Roman soldier, named Bothos, in Centurion (2010). That film also dealt with Roman soldiers warring with the Celts in Britain.