Lupin

This language consists of a variety of growls and other canine type sounds. As old as the werewolves themselves, it can be spoken while in wolf and human form. Some druids and other folk that live in the harsh environment of the wild learn the language to have a better understanding of nature and create an understanding between them and the predators that would attack their flock.
Due to how the language is spoken, it is very hard to learn by non-wolves, though. One settlement of people in the wild may have one that can speak Lupin. The smaller groups often ask for these people to travel to their towns for their services when it is wanted or needed.
Lupin is the main language or most Werewolf packs. Once a member has fully been accepted into a pack, they are expected to speak it when communicating with other werewolves, especially the elders. It is accepted, though, that one doesn't have to speak it when in the company of others, to avoid revealing themselves as werewolves to those that don't need to know.