I used Bing translator to convert your text back to Welsh* and then searched for the lyrics which gave me the result that they come from a Welsh language hymn Goleuni [Light] written by the great hymn writer, William Williams Pantycelyn (1717-1791), and published in 1762.
You can view digital texts of all his publications online through the National Library of Wales website The Printed Works of William Williams Pantycelyn, but I couldn't trace this particular hymn, probably because the text has been corrupted after so many translations back and forth!

* [The machine translation of your text came up with this:-
Mae ei eiriau'n rhoi Goleuni a chryfder, melys fel y gwlith mêl,
Cryf fel y bryniau aruthrol, Marwolaeth Crist yw ei sel,
Byddan nhw'n cryfhau fy nghwrs, yn y wlad ddiffrwyth, ddryslyd,
Byddan nhw'n cryfhau fy enaid bregus, mewn dwr a than,]