The Ardalambion indicates that it’s Sindarin, and provides a translation of this:
A Elbereth Gilthoniel o menel palan-diriel, le nallon
O Elbereth Starkindler from firmanent gazing afar, to thee I cry
sí di-nguruthos! A tiro nin, Fanuilos!
here beneath death-horror! O look towards me, Everwhite!
It continues with JRRT’s own translation(s) of this, given as
"O! Queen who kindled star on star, white-robed from heaven gazing far, here overwhelmed in dread of Death I cry: O guard me, Elbereth!"
"O Elbereth Starkindler from heaven gazing-afar, to thee I cry now in the shadow of (the fear of) death. O look towards me, Everwhite."