The Stillpine tribe is an Alliance-Template:Friendly tribe of furbolgs inhabiting the draenei areas of the Azuremyst Isles led by High Chief Stillpine — who can be found at Stillpine Hold. The existence of the Stillpines seems to contradict the point that the Timbermaw tribe is "the last uncorrupted furbolg tribe in the world". However, due to their semi-isolated location, it is possible that the Timbermaws simply were not aware of the Stillpines' existence, or at least not aware of their escape from corruption.
In-game
Initially, most draenei will not be able to understand the Stillpine furbolgs they encounter. To communicate with them you must speak to Cryptographer Aurren (who is accompanied by Arugoo of the Stillpine) in the center of Azure Watch. As part of a quest chain starting with the quest 
[8] Learning the Language, you will have learned to speak the language of the Stillpine furbolgs by the time you complete the quest [10] The Prophecy of Akida.
It is not clear if you can speak to other furbolgs, not of the Stillpine tribe, after learning their language, but it does not appear as a new language skill after you complete the quest chain.