Since our dear Hajime Isayama didn't give much information about the world of Attack on Titan prior to the manga's events, I felt free to get inspiration from other sources, both historical and fictional, to craft a setting for this fanfic. This extra chapter is basically just an addendum to help the readers understand more about this fanfic.
People and cultures
As we have seen on chapter 122, Eldia seems to have its origins on a Germanic-inspired tribe. Scandinavia, Denmark, and Germany were the bases for the names and appearance of the characters. On the same chapter, we had a glimpse of ancient Marley, which was seemingly based on Rome. This fanfic extrapolates on this idea and gives Marleyan characters typical Latin names, appearance, and culture. By the time of Ymir Fritz's death, the king, their daughters, and other secondary characters can be seen wearing Greco-Roman clothing, leading me to imagine that Eldia adopted aspects of the Marleyan culture during the process of conquest, which inevitably led to cultural exchange. In turn, the Paradisians are based on the Celtic peoples of the British Isles, especially the Gaels. They only recently begun interacting with Marleyan refugees and with Eldians due to skirmishes around the Marleyan territories and have only seen non-transformed Eldian armies until now, which explains Cliodhna's mistrust of the Marleyans' claims on chapter 15. This is simply something I thought up, as we are given no information about Paradis before the mass immigration of Eldians after Karl Fritz's vow.
As for the rest of the world (IF I get to write about it), we're not given much information as well, but an Ottoman Empire-like nation (Mid-East Alliance) and a Japan-like country (Hizuru) exist, so I assume the Attack on Titan world is, or at least used to be, largely similar to our own, only with a different geography. Other distinct cultures are also briefly seen during the rumbling but were reduced to a few panels.
Politics
Eldia is technically ruled by the king/queen (or for a better word, emperor/empress, words that Isayama never used) whose power is legitimised by his/her possession of the Founding Titan. Other clan members also have prestige due to being candidates for the succession and for their political power stemming from high positions. In other words, the ruling monarch gives out titles/positions to fellow clan members. There's a Fritz in every court, always overseeing the turning of Eldia's gears and either reporting noticeable happenings to the Founding Titan or fixing eventual problems by themselves. Such is the case with Eckhart Fritz on chapter 12 of this fanfic.
The power is not entirely centred around the king/queen. As seen since chapter 3, they have councils in which they discuss major decisions, so the power is distributed between the nine clans. The king/queen has no absolute power (yet) and I imagine a revolution would occur later to centralise the power around the Founding Titan, but this is only a deduction. Each clan has a territory both outside and inside the capital, with dynamics working similarly to margravates, dukedoms, counties, and baronies. Besides the "Empire" category, which encompasses all of Eldia and is ruled by the Founding Titan, the greatest administrative unit is ruled by the holder of one of the other eight titans, which is in turn composed of other lesser units, ruled by branch family members. The lesser units pay tribute and provide manpower for the greater units. Think of it as a village serving a duke that "owns" the region, but this duke is ruled by a king.
This means the other clans have a quite high degree of autonomy, but certain decisions can be turned down by the council or by the king/queen themselves, most of the time by threats in this latter case. We can see that Sigrid Tybur openly mocks Hildegard Fritz on chapter 14 due to her lack of experience and prestige because she knows she can get away with it. She respected Anders Fritz, but his daughter is not like him. Wars of aggression are almost always declared by the Founding Titan's holder due to their control over titans, which are the most effective weapons of the time. There are also conventional armies composed of foot soldiers, archers, and cavalry, mostly used on defencive battles, skirmishes, or raids against neighbouring peoples. Most of these these armies are composed of non-Eldians.
Eldians are largely believers of a "manifest destiny". Led by nine deities made flesh, they think they have all the right to conquer the world. This leads to severe problems in diplomacy. Non-Eldians often think of them as inhuman, monsters, demons, and the such, while Eldians think other peoples are lesser humans who long for their civilising might. By spreading the blood of Ymir among the conquered peoples, enabling control by the Founding Titan in the process, cultural assimilation becomes far easier. The number of "true" Eldians among the transformed pure titans is actually quite small, with half-Eldians being mostly used instead. Still, there's a sizable population of conquered peoples living within the empire, and Eldians are outnumbered due to their massive expansion in a relatively short amount of time.