Sylvan

Sylvan is a language one encounters in very old songs and poems about the natural world. It is the mythical language of fey creatures, and legend says that beasts can understand it. Some scholars have made a study of it, but no one in the known world speaks it as their first language.
Most Elves have at least a passing familiarity with the language, and those with proficiency in Nature can generally understand Sylvan, even if they can't speak it.
Druids can understand Sylvan, even if they can't speak it.