Eldian language

Eldian (エルディア語) is the official language of Eldia, Marley, and many other nations around the world.

Description
As a result of the old Eldian Empire's imperialism, Eldian is a known language throughout the world. It is spoken both inside and outside the Walls, although their writing systems differ. Ambassadors from across the globe used it to communicate with Willy Tybur during his pre-party.

Accents vary, as Armin Arlert says that if they are to go to Marley, they would have to pay attention to the way they talk. The citizens of Dauper have their own accent, which Gabi Braun likens to Southern Marley's.

Writing
There are two known writing systems for Eldian.

Inside the Walls
This system of writing is found only inside the Walls. The writing is composed by short, straight strokes and sharp corners, and written horizontally. It makes multiple appearances:
 * The sign of a toolshop.
 * A signboard next to the boulder used to seal the gate in Trost District.
 * A signboard of the tavern where Levi and Kenny were fighting.
 * A book that Frieda was reading to Historia.
 * A newspaper that Armin was reading.
 * Bottles of Titan injection.

Outside the Walls
Another system of writing is found when the Survey Corps settled in Utgard Castle after searching for a supposed breach in Wall Rose. They found supplies that were left behind by unknown occupants, but the labels found on them were unreadable to most of the soldiers. When Ymir went off to find food, she found a can with a herring label on it which she proceeded to read out loud. She gave the can to Reiner Braun, who was shocked to find out that she could read the writing. It is later revealed that both Reiner Braun and Ymir came from Marley, confirming that the writing is the system used by the Marleyans and the Eldians living outside the Walls.

Ancient Eldia
An archaic version of Eldian is found in ancient manuscripts and texts that were sent by Eren Kruger to the Eldian Restorationists. The language appears to include a lot of visuals and was used by the Eldians from a long time ago.

Although it was unreadable to the Grisha Yeager and Restorationists, he and his fellow patriots assumed that the texts confirm their beliefs that Ymir Fritz had brought wealth to mankind, and that the Subjects of Ymir are the "chosen children of God."

Trivia

 * The official English release translates both 「エルディア語」 (Erudia-go, lit. "Eldia language") and 「エルディア人」 (Erudia-jin, lit. "Eldia people") as "Eldian."
 * Eldian is currently the only language spoken in Attack on Titan thus far.
 * The popularity and spread of Eldian is similar to English. Their respective empires also conquered a large portion of the known world at some point.

Inside the Walls

 * At first glance, the writing system found inside the Walls may look like unintelligible gibberish, although it is actually Japanese katakana turned upside down and messily written to make it difficult to read. By turning the text 180 degrees and some effort in the interpretation of the characters, the writing can be directly translated to Japanese.
 * The texts seen in the newspaper Armin was reading and the book Frieda was reading to Historia are Easter eggs added by Hajime Isayama's assistants which have nothing to do with the context of the story.
 * The inside covers of all manga volumes include pieces of the Walls' writing revealing some of the supposed history of humanity.
 * Ilse Langnar's notebook is also written in Japanese katakana turned upside down; as well as the word Eren wrote while in Titan form during Hange Zoë's experiments. However, unlike the rest of instances of the Walls' writing, these texts are written right to left, in a style that resembles that of the writing system outside the Walls.

Outside the Walls

 * Although the writing on the wine bottle is simple gibberish, the writing on the can of herring is Japanese katakana turned upside down and slightly altered to make its reading difficult and give the impression that it is a different language. However, unlike the writing found inside the Walls, which is read left to right (like regular Japanese), the writing on the herring can is read right to left.
 * The Attack on Titan Guidebook clarifies that Reiner Braun could read the herring can like Ymir, but he lied in order to protect his own identity while forcing Ymir to reveal hers.