From the Iron Wind:
- The Fustir-Gost speak Fustiri, which is an offshoot of Umlian.
- Kayvis is the principal tongue of the Ky'taari, though most have at least a working knowledge of Umlian and Erlini.
- Amongst themselves the Udahir converse in their native tongue, though some, particular the Seers, are still taught tongues of other peoples with which the Zjedrahir (the racial group to which the Udahir belong) had contact, such as Elves.
- No language info is given for the Syrkakar.
So I think the answer is that they all speak local languages. Umlian, which is not mentioned in other books could either be a dialect of Rhaya but I would treat it more as a separate language originally developed by the Umli (obviously) with dialects based on regions, as the Umli are found on several continents.