Mary Hines points us to an old Celtic poem entitled "CAD GODDEU - The Battle of the Trees." It was used for the sanskrit choral passage in the "Duel of the Fates." This is confirmed by the latest issue of The Star Wars Insider Official Magazine, in an interview with John Williams. Here's the translation in English, and be sure to read the article on Williams for more insight into these words:

The tops of the beech tree have sprouted of late, are changed and renewed from their withered state.

When the beech prospers, though spells and litanies the oak tops entangle, there is hope for trees.

I have plundered the fern, through all secrets I spy, Old Math ap Mathonwy knew no more than I.

For with nine sorts of faculty God has gifted me, I am fruit of fruits gathered from nine sorts of tree -

Plum, quince, whortle, mulberry, respberry, pear, black cherry and white, with the sorb in me share.

From my seat at Fefynedd, a city that is strong, I watched the trees and green things hastening along.

Retreating from happiness they would fein be set in forms of the chief letters of the alphabet.

Wayfarers wandered, warriors were dismayed at renewal of conflicts such as Gwydion made;

Under the tongue root a fight most dread, and another raging, behind, in the head.

The alders in the front line began the affray. Willow and rowan-tree were tardy in array.

The holly, dark green, made a resolute stand; he is armed with many spear-points wounding the hand.

With foot-beat of the swift oak heaven and earth rung; "Stout Guardian of the Door", his name in every tongue.

Great was the gorse in battle, and the ivy at his prime; the hazel was arbiter and this charmed time.

Uncouth and savage was the fir, cruel the ash tree - turns not aside a foot-breadth, straight at the heart runs he.

The birch, though very noble, armed himself but late: a sign not of cowardice but of high estate.

The heath gave consolation to the toil-spent folk, the long-enduring poplars in battle much broke.

Some of them were cast away on the field of fight because of holes torn in them by the enemy's might.

Very wrathful was the vine whose henchmen are the elms; I exalt him mightily to rulers of realms.

Strong chieftains were the blackthorn with his ill fruit, the unbeloved whitethorn who wears the same suit.

The swift-pursuing reed, the broom with his brood, and the furse but ill-behaved until he is subdued.

The dower-scattering yew stood glum at the fight's fringe, with the elder slow to burn amid fires that singe.

And the blessed wild apple laughing in pride from the Gorchan of Maeldrew, by the rock side.

In shelter linger privet and woodbine, inexperienced in warefare, and the courtly pine.

But I, although slighted because I was not big, Fought, trees, in your array on the field of Goddeu Brig.

Expect an update on this with more information soon!