If you are a fully qualified school teacher then I guess you should be able to find a job at an international school. If all you want to do is teach English to kids for a year or so then your qualifications and experience as a primary school teacher will be more than enough. I know several...
You need a degree to get a teaching visa in Japan. If you already have a visa- working holiday, spouse etc then you can find employment teaching without the need of a degree.
Also, the degree need bare no relevance to teaching or English. Just so long as you have a proper university level...
It depends on where you want to work and how serious you are about it. If you want to make it a career then I'd recommend you get the best qualification you can. If you only want to work for a year or so then get the cheapest option, although there are still places (in Japan at least) that...