Gutterspeak

Undead speak Gutterspeak (known as Forsaken in World of Warcraft: Cataclysm) 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 (see below).

In the MMO, undead players can speak Gutterspeak and Orcish. The name of the language was changed to "Forsaken" in the expansion Cataclysm.

Forsaken Names
Like their dark lady, Forsaken keep the names they had when they were alive. These tend to be human names as most Forsaken were human. If a Forsaken cannot remember their name, they may make up a suitable name or simply take one from a headstone. Some invent surnames that imply their desire to eradicate the Scourge.
 * Male Names: Roberick, Magan, Danforth, Lansire, Aberforth.
 * Female Names: Yellen, Limmy, Sarias, Mierelle.
 * Family Names: Dartfall, Blacksling, Ghoulhunter, Blastlich, Gutripper, Antonovitch.

Sample Words (speculation)
This is the list of words created by the in-game language parser for the Common language, and is listed as language number thirty-three (word range 1371-1494) in the Language text file.

In the in-game translator, the language parser for Gutterspeak shares similar words with Common and Gnomish.

Note: The language algorithm used by the in-game "translator" merely makes the words LOOK like Gutterspeak. It does not actually use a specific dictionary. Therefore, translated in-game speech isn't true Gutterspeak.

Language Barrier
It has not been explained within the context of the game why the Forsaken in World of Warcraft are not able to speak or understand their language during life, Common. However, according to lore found in the Horde Player's Guide undead Forsaken have indeed not lost the ability to understand Common, but just never choose to speak it under normal circumstances.

The language barrier is simply a game mechanic, and no attempt needs to be made to reconcile it in terms of lore. The inability of Forsaken to speak and understand Common is not found in other sources. According to the RPG, Forsaken learn the languages of their enemies and their allies (who may soon become their enemies). Thus, they are often fluent in several other languages besides Common and Gutterspeak, including Orcish. The Forsaken encountered by the main characters in The Sunwell Trilogy are also capable of speaking Common.

Game Mechanics
The real-world explanation for the fact that undead aren't able to speak Common is simply because the developers don't want players communicating across factions. In the Beta, the undead player characters spoke Common (besides Orcish) and thus could communicate with Alliance characters. This led to a huge amount of vitriol and bile that was spewed both ways, so this functionality was removed and Gutterspeak was introduced.

In the game, neutral undead NPCs are capable of speaking to the Alliance.

It has been theorized that Common and Gutterspeak are translated to Orcish in the exact same manner, although this claim is disputed. Those disputing it say that while "lol" translates to "bur" in Orcish from both Common and Gutterspeak (along with several other tested phrases), these are similarities purposely put in because undead were once living and thus once spoke Common, and have since then changed their language somewhat. It is more likely, however, that when Blizzard wanted to create a language barrier, they left undead speaking the same language, but gave Gutterspeak the same translation scheme as common to both Orcish and Common as Common has to Orcish, thus creating a barrier, but removing the need for explaining why they speak "different" languages. This is corroborated by the translation filter using identical 124-word long sample word lists for showing Common and Gutterspeak to non-speakers.

Fan Speculation
Dan Golthas: "When I clawed my way out of the grave, I thought my family would welcome me with open arms. Instead, they drove me out of the village, screaming in a language I could no longer understand."

Some fans who have tried to reconcile it in their own way have theorized that they just don't want or choose to talk common. Some due to their hatred of humanity, others because they wanted to break their ties to their preceding life (some RPG books support this theory), and some, like Dan Golthas, who have "forgotten" how to speak Common for some reason.

It may be because some Forsaken exhibit signs of old age, losing their identities and eventually their sentience. They appear feeble and scared, and forget who they are. They are losing their mind to the Scourge, eventually become mindless zombies. Likewise, it is possible that when something is killed and raised as a undead, memories of things like languages, names and other similar information is wiped out as the only thing a servant of death is to hear is the hypnotizing and unceasing whisperings of the Lich King. Then again, this would not explain how Forsaken knows Gutterspeak and Orcish when starting the game.