Explaining the Lyrics To Star Wars Duel of the Fates

So what were they singing in "Duel of the Fates" ?

It's a line from a Welsh poem, Cad Goddeu ("Battle of the Trees"), originally translated by Robert Graves, and literally transcribed into Sanskrit. The line goes, "Under the tongue root a fight most dread, and another raging, behind, in the head."

Sanskrit lyrics?
Williams chose Sanskrit because it was not as recognisable as Latin or Greek, and the vowels were easy to sing. The lyrics are:

Khara Matha Khara Rath Amah
Khara Rath Amah Yuddha Khara
Khara Syada Rath Amah Dai Ya
Khara Ki La Dan Ya
Niha Ki La Khara Rath Amah
Syada Ki La Khara Rath Amah
Khara Dan Ya Khara Rath Amah
Khara Dan Ya Khara Rath Amah
Niha Ki La Khara Rath Amah
Syada Ki La Khara Rath Amah
Khara
Khara Matha Khara Rath Amah
Khara Dan Ya Khara Rath Amah
Niha Ki La Khara Rath Amah
Syada Ki La Khara Rath Amah
Khara

Was it then also Sanskrit at Qui-Gon's funeral ?
Yes. Before you ask:

Mahdhurah swehpna
Go rahdomah swepna
Mahdhurah swehpna
Go rahdomah swepna
Moorittioo mahdurah swehpna

What do they mean ?
Good question. This is an approximate guide. Don't worry if the words don't translated too well in English - because there is no formalised grammar in Sanskrit.

Khara dreadful
Matha head
Rath speak
Amah give
Yuddha battle
Syada raging
Dai purify
Ya going
Ki like
La taking
Dan separate
Niha loss
Go rahdomah to set out for battle
Swepnah, Swapna dream, dreaming
Madhura, Madhurah sweet
Moorittioo death