I like the concept and agree that it will make the setting very immersive, however there also needs to be a mechanism for a character to be able to solve and use the cipher even if the players can't work it out. Otherwise it will just create frustrations for both players and GM. From my perspective I would want to see clear examples of how the cipher works with magic etc as I don't want to spend a lot of prep time creating these puzzle pieces.
I don't think you need to worry, Lupex. The thing is that the Maha language is just a cipher once, as a puzzle in a specific location. Once you grasp it, it's pretty straight forward, and then it's more a question of using it creatively. At that time, the GM might let the players read the appropriate section in the rules. It could be a good idea to let the druids use magic in a standard way and leave this language as an option for those who like the concept and challenge. It's kind of hard to discuss it before it's around.