I'm not sure that the Craftworld Eldar would let one of the Dark Kin come near enough to make themselves understood in their desires, let alone consider them. Remember that DE represent everything wrong with the Eldar psyche as far as the Craftworld Eldar are concerned.
And it's possible that they have the same language. More likely than not, seeing as harlequins freely travel between the two groups. But there must be dialect differences.
I can't find any reference to light here, but if you want some Eldar language examples, try the English-Eldar dictionary at
English-Eldar Dictionary: v2.5. From what I can tell it's more or less accurate.