Gutterspeak is the language of the Forsaken.
Gutterspeak is a lower form of Common that uses little (very little) bits of Dwarven and Thalassian. It has existed for a while (longer than the undead Forsaken have), and evolved in the shady underground of black markets and rogues' guilds as the tongue spoken by people of ill-repute. When the Forsaken took the Undercity, they adopted Gutterspeak as their official language. One explanation would be that the Forsaken, discarded and abandoned by their loved ones, found it appropriate to adopt this language of the rejects of society. They decided to keep their language from the other races of the Horde so they can converse and plan without being overheard.
Forsaken can still understand and speak Common. However, they never speak it under normal circumstances. Some have forgotten how to understand Common.
Adapted from Wowpedia.