Do you have a quote on that? Because I think that the entirety of k8p dwarf forces speak Reikspiel (evidence: Belegar gave his pre-battle speech only in Reikspiel) while in the Karak Azul contingent there is precisely one dwarf speaking it (evidence: the scene at the Karak Azul gates).
This is just a nitpick, though.
The core of the Expedition were Dwarves from the Holds with strong ties to the Empire. They've all gone home. Those that make up the Dwarven population are either Clan Hazkal, and thus from all over the place, or those that were waiting in Old Holds in hope of their ancestral Hold being retaken.