What is the song of Elfen Lied?
I presume you are talking about the theme song.
Lilium - Lily is the song, it is Latin.
The Lyrics are as follows
Os iusti meditabitur sapientiam,
Et lingua eius loquetur indicium.
The Mouth of Justice is meditating wisdom,
and his tongue is speaking of a sign.
Beatus vir qui suffert tentationem,
Quonium cum probates fuerit accipient coronam vitae.
A happy man who suffers temptation,
Since he is with approval, they receive the crown of life.
Kyrie, ignis divine, eleison
Oh Lord, Fire by divine power, have mercy!
O quam sancta, quam Serena,
quam benigna, quam amoena
O castitatis lilium
Oh, how holy! How serene!
How kind! How pleasant
Oh Lily of chasitity.