Some very cool ideas here. One impracticality, though: if people are forced to speak a language that they won't understand, society will break down almost immediately. Also, the disease won't get a chance to take root if the infected are so easy to spot.
Suggested changes (includes some editing):
Abyssal Utterance
Level 3 Disease
Fortitude Attack: +6
Endurance Improve DC: 16
Maintain DC: 11
Worsen: 10 or lower
Stages of the Virus
0: Target is cured.
1: Initial Effect Target can understand Abyssal as though it were his native tongue; target suffers -2 to Will defense.
2: Abyssal Tumor grows on the host's body. This growth has sentience, and begins to affect the host's mind. Lose 1 healing surge. Whenever the host becomes stressed, he will subconsciously begin using some Abyssal words. This happens automatically when the host attacks or makes an Intimidate check.
3: More, powerful tumors spread. The host takes -2 to Fort. The Tumors begin influencing the host more, subconsciously encouraging the host to spread the virus to as many uninfected individuals as possible. Host will engage in fights and seek occasion to make Intimidate checks. Host will also refuse to partake in any Cure Disease or Remove Affliction ritual.
4: Final Stage The Tumors' control of the Host's will has become complete; an entity that can possess targets does not need to succeed in an attack to possess the Host. Furthermore, the Tumors have gained enough power to connect to the Abyss; the Host has a 15% chance per day of being possessed by a demon from the Abyss.
Contraction
Abyssal Utterance is an utterly wicked plague, not only due to what it does to a victim, but how it is spread. The first external symptom of the virus is that the host speaks Abyssal. Anyone who hears the infected host speak Abyssal is exposed to the virus.
- - -
Why this is better than your version: it's much more insidious. Consider the following interview:
Jamie: Oh yeah, Eddard was actin' suspicious three days ago. He was pickin' fights and talkin' funny. But he ain't said no more 'o them funny words since Monday.
What happened here? Did Eddard get better (which is possible), or is Jamie now in Stage 1?
Mua-ha-ha, -- N