What if someone would have an english degree but lives in a country where english isn't a native language, would he/she be seen equally to become english teacher in Japan as someone from the USA/UK/... ?
No. You are not equal in the eyes of immigration. Can you clarify what you mean by an "english degree"? Is that a degree in the major of English, or a degree in something else but taught in English?
Basically, non-native English speakers have to show to immigration that they have 12-13 years of their education solely in English before they can get a work visa. You'd be surprised at what countries with English as a second language are not counted by the Japanese as native English speakers, so essentially, those that ARE counted are usually only the USA, UK, Canada, Australia, and New Zealand.