I'm native German, but trying to write in English for various reasons. One of the big points in a story is to keep the characters very distinct from each other, and that requires some knowledge about the culture. As said, I'm native German, so it's easy for me to envision a character that comes from Cologne and how different that one would be from someone who comes from Bavaria, Hamburg or Berlin.
Are there resources for various countries? For example, If I wanted to know how an American from Arkansas is different from someone in Southern California, or if I have two Serbian characters that come from different backgrounds and no idea what realistic backgrounds for Serbian people are (Is Niko Bellic really a believable Serbian or is he just a summary of eastern european cliches that westerners have?)
I'm guessing there is no easy answer for this since culture has to be experienced first hand to truly understand and "feel" it, but I still wonder if there are known and trusted reference books or website that summarize some truths and false myths, that characterize people's accents and typical word/slang usage and just give an overview, preferably with some real existing people that fill these stereotypes and that can be studied.