Foreign teachers are very common here. One of the jobs that foreigners (especially English native speakers) can get easily is teaching. I don't know where you would like to teach (elementary schools, middle schools, high schools, private English schools, universities, ... etc.), though.

Language teaching is a popular job in Japan for foreigners. The difficulty for you may be that the most "popular" foreign language for Japanese to learn at schools and language schools is English, and that they normally require native speakers of English as one of the qualificatioins.

When it comes to Portuguese, I don't know how much demand there may be - language schools (when I say language schools, I mean privately-run companies that offer courses in different languages, mostly to adults) advertise English, French, German, Italian, etc., but I don't remember seeing Portuguese mentioned in the advertisement commonly seen.

Another thing you might want to think about: teaching a language and doing translation are two quite different things. And, in addition, translation (normally written) and interpreting (oral) are two different things. If you are good at acquiring multiple languages, and like communicating/interacting with people, you might want to consider becoming an interpreter in Europe? For major international conventions, they do require a lot of capable interpreters.

While there is some demand for Portuguese, I'm not sure if you have heard this before, but in the early 20th century a lot of Japanese people migrated to Brazil. Now many of their descendents have come to Japan to work, and they are often Japanese citizens and native speakers of both Portuguese and Japanese, so they tend to take up the jobs where Portuguese is required.

I'll consider everything you said from now on and start looking at all the options I have, while searching for new ones. Perhaps staying within Europe would indeed be easier on me, and I can always visit Japan in my vacation time.