Margaret's done a cracking good job, although she PMed me to get some feedback. I've made some very minor corrections (all CAPS) and added the last verse.

Drylliwch y cyffion, daeth yr hyfryd ddydd Fe ddaw gweithwyr Cymru yn weithwyr rhydd.

Shatter the chains, the beautiful day will come When the workerS of Wales ARE free workerS

Dewrion y lofa a'r peryglon mawr Fechgen y chwarel, daeth eich awr Dowch o'r bythynnod i'r fuddugol gad, Y mae chwyldroadau'n siglo'r wlad.

The brave mEn [OF THE COALMINE (lit. coal place) ] and of the places of great dangers, Young men of the quarry, your hour HAS come [COME ] from the cottages to the victorious BATTLE, Revolution is shaking the country.

Tyrfa'r heolydd, prin eu bwyd a'u gwaith, Swn buddugoliaeth sy'n eu hiaith; Y mae'r deffro'n cyffro tyrfa'r stryd, A gwerin Cymru yn arwain byd.

The multitude on the roads, scarce their food and their work, A victorious SOUND IS THEIR LANGUAGE (ie. the sound of victory rings in their language / speech, depending on whether he's making a point about Welsh as well, which, considering the poet, he probably is) Excitement is awaking -- the crowd on the street, And the people of Wales are leading the WORLD.

Disgyn gwrhydri'r tadau ar y plant, Esgyn gobeithion y byd o'r pant Anthem y Chwyldro sy'n atseinio'n groch, Un yw gwerin byd dan y Faner Goch.

The valour of the Fathers descends on the children, The hopes of the world rise up from the valley, The Revolution's Anthem fiercely resounds , The workers of the world are as one under the Red Flag.

sian