Beast Tongue

Like the Skaven, Beastmen have a difficult time forming the words of the Dark Tongue with their malformed bestial maws. As a result, they use a crude mixture of Dark Tongue, body movements, grunts, clicks, and pops, collectively called the Beast Tongue. This language sounds more like noise than a sophisticated tongue, being not much more than an indescribable muttering and grumbling. Add to this the shrieks, howls, and bleats that Beastmen use to punctuate and emphasise their phrases and there’s considerable debate whether Beastmen speak a language at all. But regardless of prevailing opinions, Beastmen are an intelligent race, and despite their brutish aspect, and the fact that some of their kind cannot speak at all, they use their Tongue effectively, conveying a wide range of subjects and concepts.

The Beast Tongue is beyond Mankind’s ability to speak. In the rare instances when Beastmen deign to talk with Humans, they use a mix of basic Dark Tongue, body language, and gestures, only descending to Beast Tongue when frustrated, and killing the Human when really frustrated.

Runes of the Beast Tongue
The Beastmen use a simplified form of the phonetic runes used in Dark Tongue to mark Herdstones or to leave messages for other members of their herd. These are always crude for Beastmen lack the precision needed to write clearly. Most Beastmen use their bodily wastes instead, finding the experience far more rewarding than taking the time to scribble something on a rock.

Source

 * Warhammer Fantasy RPG 2nd ED -- Tome of Corruption (pg. 110).