While hardly a matter of great importance, I think it would be accurate and helpful if Waelan was added within the Common Languages for the Moonshae Isles background since that was the native language of the Ffolk.
This and other regions was asked about with the DMs when backgrounds were made, but DMs made the call to use the languages given in FRCS and Player's Guide to Faerun, not the additional languages provided by Speaking in Tongues in Dragon magazine. You would instead fluff the Illuskan you receive as Waelan, rather than using Waelan itself.
And yes, this does mean we should probably remove the Speaking in Tongues languages as options to avoid this confusion.
Tarina — The Witch of Darkhold, a dealer in spirits and black magic