@Kaiser92 That is not german. It is conlang made by mix up mainly german, a bit of polish and russian words but with normal latin alphabet instead of cyrillic. If you look closely at the text it is readable with broken german and polish write in broken english grammar. And if you try to translate that with any translation service it won't work because this language doesn't exist.
This manga takes some idea from real world cold war, pre-space race competition to space and a bit of german aesthetic like balkenkreuz on stuff. But I see lot of 'is this german' on official site too because how Hessinia work. Basically Hessinia is Soviet+ WWII German aesthetic. Hessinian flag is a outine of nordic cross (white part of norwegian flag) with 2 clovers, many mistake it as Kriegsmarine flag reference.
Edit : I forget to explain the invader part. It is a plot fuel to made world technology developed faster, from iron sword to jet plane in a few hundred years and a reason why Yoshka being born and hunt down from all sides. And why anything about moon and magic being seen as cursed.