Telepathy

Not specifically a seperate language, but when it comes to speaking in silence, not much can beat magical telepathy. Telepathy is not limited to any specific language.

True telepathy- that is, a connection between two minds- is extremely rare. Vampires- Blue are capable of communicating between master and child like this, due to the psychic link necessary for the transformation into a Blue. In some cases, it's been reported that twins who have gone through traumatic or otherwise magical, mind-altering experiences can potentially develop a limited form of telepathy.
However, when most Z'hratians talk about telepathy, they're actually referring to psychic projection, a form of illusion magic where a sender forces sensory data, real or otherwise, onto a specific receiver. With these expanded parameters, all Blue vampires, some faeries, and extremely talented mind wizards are capable of speaking to nearly anyone on the plane, although projection still has its limits:

• Projection works the best when the mage personally knows the reciever. Due to the targeted nature of the magic, unfamiliarity may cause the spell to "miss," hitting either the wrong target or no one at all.
• The longer the distance, the more energy-intensive and tiring it becomes, and long distances raise the probability of a miss. Most mages can accurately project a block or two, and Blue vampires have been shown to be capable of projecting a few miles.
• Unless the sender is speaking to someone with the same ability, the receiver cannot reply.

A citizen could, in theory, hire a mage to cast a projection on their behalf, but it's expensive due to how tiring it can be; even a practiced wizard would likely reach their daily magic limit to cast one. The client also assumes all social responsibility and risk in case of a miss, so for most people in most situations, it's simply not worth it. Courier services are much slower, but cheaper and more secure.
Prank calling others through psychic projection is highly discouraged, and the Occult Tactical Response Unit takes these breaches of privacy very seriously. It is one of the most minor crimes broadly defined as "mystical stalking," but will result in jail time after the third strike.