Undead speak Gutterspeak in addition to Common. 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 spoke by people of ill-repute. When the Forsaken took the Undercity, they adopted Gutterspeak as their official language. To sum up the undead's attitude: they had been thrown away like trash, and abandoned by everyone, even their friends and loved ones. Thus Gutterspeak, as the language of the outcasts, seemed appropriate to them. Note that undead can still understand and speak Common. However, they never speak it under normal circumstances. They take fierce pride in speaking Gutterspeak instead. Some have forgotten how to understand Common.
Taken from Wowwiki.
The language that undead characters can speak and understand in addition to Orcish was known as "Gutterspeak" until the shattering patch changed the name to "Forsaken".
[Screenshots containing UI elements are generally declined on sight, the same goes for screenshots from the modelviewer or character selection screen.]
[Please review our