Elven

Elven is similar enough to Sezithorum's native language to be mutually intelligible. When Maress first appeared, he spoke in the lingua franca of their world, because it's more common and widely understood, rather than Elven, which is generally only spoken by the elves – which Sezithorum clearly isn't.
Elven adds more vowels to break up the consonants, and drops some of the hard consonants completely, for a sound that's smoother and more sibilant.