For Japan, the associate’s degree probably won’t have any real value. For applications, they want the highest level of schooling you graduated from. They don’t care about anything really below that. As for whether you can get a visa, it will mostly depend on if you can get job, don’t have a criminal record and haven’t gotten in trouble with Japanese immigration in the past.
As for teaching, it’ll depend on getting a job. Trying to get a job from outside of Japan will limit your options.
-JET Program: You are young enough that you could try to come via this (which would places you in schools). I’m not sure how competitive it is in Belgium. They’ll do most of the visa process for you.
-Eikaiwa (or conversation schools, usually all ages, from babies to adults, though there are some that are kids only): most of the big companies have recruiting abroad. There is a general prefer non-asian looking foreigners. They‘ll do most of the visa process for you.
-Dispatch companies: they do school placements, but I don’t know if any/many have overseas recruiting. I think most of these companies hire people already in Japan. So for these, you could come to Japan on a visitor's visa (90-days max) and then try to find a job that will sponsor a visa for you. Then apply for work visa while you’re here. You just can’t work at all while using the visitor’s visa.
For schools, the big hiring time is March (April starts the school year), with small pushes in August and December/January
from foreigners who’ve left mid-contract/year. The conversation schools hire year round.
Hope this helps.