In the Dungeons and Dragons 5e Monster Manual, the gnoll statblock lists their language as Gnoll. However, in Volo's Guide to Monsters, the section on gnolls indicates that they communicate with
howls, whines, gestures and a broken form of Abyssal. (emphasis mine)
Is there any other documentation that explains if the Gnoll language is distinct from Abyssal, or just a subset or dialect of Abyssal?
According to Volo's Guide to Monsters (p.33), Mordenkainen's Tome of Foes (p.29) and Out of the Abyss (p.246), Gnolls are created in the wake of Yeenoghu's great rampages across the planes. Originally, they were packs of hyenas that feasted on the corpses left behind by the great Demon Lord. They now breed true, but it's entirely possible that new tribes of Gnolls could spawn the next time Yeenoghu goes on rampage. It is reasonable to believe that this demonic origin has granted Gnolls the Abyssal language.
However, Volo's Guide to Monsters (p.35), also mentions in the "Language" section :
When gnoll leaders must share complex concepts with each other, they use a broken form of Abyssal gifted to them by Yeenoghu. The gnoll language lacks a script or written form, though elite gnolls can use their limited knowledge of Abyssal to leave messages.
This leads me to believe that the Gnoll language is not a dialect of Abyssal, but rather a very primitive collection of grunts and howls, and that only Gnoll leaders and probably also Gnoll cultists of Yeenoghu can actually speak Abyssal, albeit in a broken form.
To further prove that point, Volo's Guide to Monsters (p.153-155) presents four new types of Gnolls : the Flind, the Flesh Gnawer, the Hunter and the Witherling. Of those, only the Flind has the Abyssal language in its stat block, and it is also the only one to be described as a pack leader.
