From WoWWiki

The Stillpine are an Alliance-friendly tribe of furbolgs inhabiting the draenei areas of the Azuremyst Isles, led by High Chief Stillpine. The existence of the Stillpine seems to contradict the point that the Timbermaw Furbolg are "the last uncorrupted furbolg tribe in the world". However due to their semi-isolated location its possible the Timbermaw simply were not aware of the Stillpine's existence, or at least not aware of their escape from corruption.

[edit] 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 learn to speak the language of the Stillpine furbolg by the time you complete  [10] The Prophecy of Akida quest.

It's not clear if you can speak to other furbolg 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.