Africa
Betta's originally come from Thailand, which used to be known as Siam. Hence the name "Siamese Fighting Fish".
Fighting fish is their nic-name so they should be kept separate
Japan
There are no fighting fish that come from Japan. I think you are asking about the Siamese fighting fish (Betta splendens). They do naturally have a round stomach almost like a ball. That is their body shape.
Mackerel are salt-water fish, and come from the sea not the country.
No, fighting fish (Bettas) are only interested in fighting with other fighting fish.
Of course they can. How else would we have fighting fish.
your bum
The scientific name for the Japanese fighting fish is Betta splendens.
If your fish is a fighting fish it will be still for most of the day and probably attack another fish who wanders into his "territory". Fighting fish often come in bright colours like red or blue. But some fish are just naturally aggressive and hurt or kill your other fish! {trust me I've experienced this } It's very important to know the difference between a aggressive fish and a fighting fish. If you still can't tell talk to your local pet store or talk to an expert.
Siamese fighting fish come from the tropical islands of Asia and are nothing special to China. The only connection is in the simple fact that so many people do not know their geography and have Siam, (Which is now Thailand), Indonesia and Malaysia mixed up and call the fish "Chinese fighting fish" in error.