Take the Crudely-written Log to Kelek Skykeeper in southern Felwood.
The Winterfall are no longer a threat to me, but if you still want to find out what was driving the furbolg to create and consume this strange firewater, I think you should seek out Kelek Skykeeper.
He is part of a group known as the Emerald Circle, druids that are committed to their work in Felwood, healing the land from the corruption.
Kelek has much experience with the furbolgs in Felwood, and may be able to tell you more about the log.
Thank you, again, <name>.
The Emerald Circle is friendly to all, <race>. What can I do for you?
Yes, if you let me examine this for a while, I might be able to translate it for you.
<Kelek flips through the pages.>
When I am able to look over this fully, I will send word to you.
Thank you for sharing this with me, <name>. Very little is known about the Winterfall furbolg.
After some time you'll receive [Words of the High Chief] in the mail. Which will contain the translation.
-  Winterfall Firewater
-  Falling to Corruption
-  Mystery Goo
-  Toxic Horrors
-  Winterfall Runners
-  High Chief Winterfall
-  The Final Piece
-  Words of the High Chief