They've mentioned this before but I'm confused now because Marley says that they need to be more advanced than the other nations. So are there other countries out there with like modern tech? And would those be countries in our world?
In the manga, Marley [the County that Eren's father came from and the other titan shifters] is another nation in this manga also an Asian nation from which Mikasa's ancestors come from, the Hizuru . Untill the moment, this is an alternative universe to this, so is not really our Earth or our countries. There are also other nations, but the main nations that appears to be everyone's enemy would be Marley. Marley has tried and has ruled or is currently ruling other nations. The Hizuru are joining with Paladis/Palaris [The island where the story takes place]. The Wikipedia link has the rest of the story. I'm up to update with the manga, and the Wikipedia has it pretty much right https://en.m.wikipedia.org/wiki/Attack_on_Titan