The Old World Language (旧世界の言語, Kyūsekai no Gengo), also known as Tenma Characters (天魔文字, Tenma moji) is the Language of the Fifth Heaven , now incomprehensible to the people of the Sixth Heaven. This can further be extended, as displayed by Mercurius, to simply be the form that the tongue of a pervious Era of the Throne takes on where perceived by those of a successive era.
This can be seen as an extension of the incomprehensibility of a previous era to those that were not born of it. Which one sees in many different works, from the bizarre and anomalous nature of Sensories, to the incongruity with existence of the Singularity Angra Mainyu, and many more examples which display this idea of incomprehensibility.
Mechanics[]
It is spoken by the Yatsukahagi, leaving them as noisy and untranslatable. It is also spoken by Mercurius in Dies Irae when he activated his Atziluth.
Though once you finish the Grand route, you can replay the game and see the translation of their Dialouge.
Trivia[]
The characters it is written in are Ahiru Kusa Characters, a type of Kamiyo Moji. This script is undeciphered and little is truly known or understood about it, and much debate exists today around their authenticity.
This could be connected to the idea of the incomprehensibility of the Age of Zero, and the idea of the world of a previous God being not understandable by those bound to the era of another. This, however, is simply a theory made by myself, do not take this as fact.