Indian Language
Having your posse members spend months learning the Sioux language isn't really going to stand them in good stead when in Apache country, so knowing the correct language for the correct area is a must if you want to get along with the locals
There were actually dozens of languages spoken by Indians, which is why trade and co-operation was sometimes difficult, but I have broken it down into 4 main languages - since the Reckoning occured and tribes have been able to unite they have melded their languages together as follows;
Indians of the Sioux Nations and the great plains consisted of various tribes native to the plains, and those from the North West and as far away as Canada, they now collectively speak one language called Siouan
The aptitude to speak this language is Language:Sioux
Indians of the Southwest, the Apache and Commanches et al, also spoke various languages but now all use a common tongue - the Apache language of Athapaskan.
To speak this tongue one must have the Language:Apache aptitude
As far as the Coyote tribes go their languages were often varied - attracting tribes from the East coast and South east of America. They all still speak their native tongues but all also know the Coyote language of Algonquian
Speaking this language requires the Language:Coyote aptitude
The Indians of California were the most diverse of tongue, several dozen languages were spoken in this region alone. For this reason the Necessity Alliance of the Maze Indians have taken to speaking a single language (in addition to their own native tongue) called Necessity Talk. To understand this one must have the Language:Necessity Talk aptitude
In Mexico (and to a certain extent in the Southwest America and New Mexico) the local dialect is called Uto Aztecan and is sometimes referred to as the Sacred Language. All American Indian tribes using this language certainly also speak another (either Coyote or Apache, but the Mexican tribes such as the Concho and Aztec would certainly have this as their first language.
To speak the sacred language one needs Language:UtoAztec
Not forgeting the famous silent language used by Indians, all tribes have their own but if you know one it can be assumed you know the basics of all (perhaps at a -2 modifier to rolls to "sign" to an Indian of different native tongue). This Indian Sign Language is used for hunting purposes, in front of white men, and when setting ambush - conditions when silence is needed. It isn't too hard to pick up and anybody wanting to learn this aptitude (Language:Indian Sign) does not need spend double points, but it does help to have a teacher. Finding one that is willing is another matter.