It's specifically Orcish, which is kind of weird (I would think there would be one Greenskin, or Orc and Goblin tongues)
It's referred to as 'Orcish' by the Empire because they're only exposed to two Greenskin languages and the other is the Forest Goblin language. A better term for it would be 'Old World Greenskin', which differs from the original or 'Savage' Greenskin language mostly from the influence of Chaos Dwarf Khazalid.