Good respectable elven children learn Sylvan. Everyone in Syngorn knows that.

Vax and Vex sign up for Abyssal lessons, enjoying the way the adults around them flinch at the scratchy hissing; at night, when Vax sneaks into Vex's room and they pull the blankets up over their heads, they whisper to each other in Common, Mother's lopsided accent stretching their vowels out.

(They stop when Vex realizes, months after the fact, that it's gone, grinded down to dust after years of living among elves; Vax sounds almost like Syldor, now.)