I don't think that "go south" has anything to do with the civil war. I think it's simpler: on a standard map the direction south corresponds to down. So going south just means going down. For example, on a graph of a company's profits, if the line is going south then the profits are going down.

I would definitely say yes. And I feel like these expressions are not even specific to a certain language, but rather to a country, or even just a region. For instance Australian idioms I would imagine are probably disparate from American ones. And I definitely think idioms are the hardest part to master in any language and can be the most hindering in effective communication between native and non-native speakers. Like I can understand most of the stuff my teacher say in a classroom, a rather formal setting. But in daily life, or say on the internet, I often find myself having to look things up on the internet despite living in the U.S. for 2 years. The same goes for Chinese: there are tons of this thing called 成语 which are succinct phrases derived from ancient fables/historical events(sometimes it's impossible to tell lol) and we are required to study them in schools because it's just such an essential part of the culture, even though we don't use them nearly as much in daily life. And the absolute most interesting thing I've found is how the same thing can be expressed in completely opposite ways across different cultures. For instance in English when something is failing you'd use the expression "go south" which I believe is an allusion to the civil war??? maybe?? But in Chinese when someone loses, u'd say 败北 which literally means "lose north". So yeah.. idioms have just never stopped blowing my mind..