Spokuruna
Spokuruna originates in Latvia, mostly by accident. The country has been fought over several times over the last century or so, invariably by regimes nasty in word and deed, which means that there have been a lot of people killed suddenly, and with unfinished business. As anybody familiar with spectral phenomena can tell you, that’s an excellent way to get ghosts. And that is exactly what has happened in Latvia, as it has happened throughout human history. It mostly doesn’t matter, as most ghosts can’t be seen by anybody who isn’t Sensitive in a particular way, anyway.
What was different in Latvia was that in 1978 a local Sensitive made contact with a local ghost, and it happened that both of them understood Latvian Sign Language (which is a variant of either French or Russian Sign Language, depending on who you ask). This has happened in the past, certainly; but only a few times, and generally nothing came of the efforts. However, in this particular case both the Sensitive and the ghost were equally determined to spread the language among the relevant populations. By now, Spokuruna ‘speakers’ can be found across Central and Eastern Europe, and a hard core of American spiritualists are simultaneously trying to learn and teach it on their own continent.
The language is called ‘Spokuruna’ (to distinguish it from Latvian Sign Language) because the signs have been heavily adapted and stylized to take into account the needs of the spectral and spiritualist communities. It’s excellent for making bargains and describing specific metaphysical situations and dangers; reasonably good for describing emotional states; and almost useless for discussing current affairs. Many ghosts have difficulty focusing on mundane events anyway; long experience has shown that teaching the signs that would properly narrate, say, a soccer match would be effectively pointless. But for what it’s good at — allowing a Sensitive to negotiate with a ghost over ending a haunting — Spokuruna is very good.
One last note: there is no need to learn Latvian to learn Spokuruna. Most sign languages have only a vague relationship with the local spoken tongue. But knowing a dialect or two from the French Sign Language family would probably be profitable.