How we do research is a case by case basis, really.
In the case of cultures we're not very familiar with like Themba or Khenbish's we read about the country, its history, watch documentaries and movies, and most important of all, try to talk to locals. We know a guy from South Africa (who's working on Lord of the Manor, an African-inspired visual novel) to consult for Themba, but as hard as we tried we couldn't get a reliable Mongolian person to interview for accuracy. We did have a friend of a friend who studied with a Mongolian guy and he answered a lot of our questions, so that helped.
We didn't really interview Greek or Japanese people for Kota and Asterion since people from the Minoan civilization and Meiji era Japan aren't around to ask (this might change for Nikos since he was actually born and raised in contemporary Greece, though). For them we mostly rely on historical research from a lot of sources. Kangarube (who writes most of the poetry for the game and does a lot of editing) has an arts degree, regularly reads ancient Greek plays and poetry so he's our main consultant. Minoanon also read up on texts like the Homeric Hymns and that helps a lot.
As for Brazil, well, Minoanon is literally Brazilian.
Personally I really like small quotidian aspects of the cultures we portray because they make for fun character interactions. Stuff like Khenbish cooking local recipes for the guests, I love doing research on that. There's a scene that will go into one of the next builds where the main character can pick drinks for Kota to serve him. If Themba is available, one of the shots you can take is a Springbokkie, which has an amusing drinking ritual around it he makes the main character partake in, that's the kind of stuff I love to put in the game.