Is WIQL specific to the TFS SDK or can it be used in other languages?
For example if I have built a client using TFS SDK that gets info about projects on the tfs, is it possible to somehow reuse the WIQL if I want to build a javascript client that does the same thing? Or is WIQL only used internally by the tfs sdk to create the proper soap request?
Your WIQL is processed on the TFS App Tier so, yes, you could conceivably write a Javascript client that sent WIQL queries to the server (in the context of a properly formed SOAP call). I wouldn't recommend it, however. The SOAP API for work item tracking isn't conducive to that approach.