That will happen if you go anywhere where the spoken language isn't your native tongue. the brain is capable of picking up and learning new things all the time. it starts with you being able to understand what people are saying without necessarily understanding the actual words. then just goes from there, it takes time though, how do you think babies learn to talk?
Theoretically, yes it's possible..However, the probability of someone doing it (whom is not a child and knows no Japanese) is slim to none. The reason it works for younger kids and babies is because from age 0-9 a child's brain is basically plastic mot, therefore the rate at which they pick up things are extremely accelerated compared to someone older.
With no previous knowledge at all of Japanese, I strongly believe it won't be possible to strive in an environment where one doesn't even know the grammar of the native tongue being spoken.
However, if you have proper knowledge of grammar and have taken as short as a year's study in Japanese, your knowledge and ability to comprehend and add to that comprehension will soar when you go to Japan.
If you're looking for a working proficiency (and even perhaps fluency) in Japanese, take a Japanese language course while staying in Japan. Total immersion is the best way to learn.
Or.. instead of looking for a Japanese spouse in particular... (rolls eyes)
There are certain periods of time in a person's life when you can learn a language. The older you get, the harder it becomes to learn. So the best thing is to start young when your mind hasn't slowed down, or take an intensive language course when your older.
In theory, it's not possible by immersion (according to sociologists and psychologists) because the mind is set up to learn language at certain stages in life. Once that time has passed, it's near impossible, at best difficult. Of course that time is different for everyone.
SacredBlue made a good point of mentioning "I strongly believe it won't be possible to strive in an environment where one doesn't even know the grammar of the native tongue being spoken." When I was taking Spanish from 1st grade to 7th and stopped all together, I noticed that after knowing all the declensions, tenses, etc. it was only a matter of grammar. Ergo, learn you grammar! ども。
By the time you could live in Japan(realisticly, once you graduate from college), learning from straight immersion and nothing else wouldn't be so successful. But that shouldn't deter you at all. Simply spend time wisely until then and study Japanese. I'm sure you can find a good tutor or course in your area to get you started. The 5 or so years you have to study should be more than enough time, and I'm sure many gaijin in Japan would love the chance to have such an opportunity. Good luck
It really all depends on the person, and what you consider the basics, and how much you study, etc.
For what I consider basics, I would say it'd take a year to master. When I say study I mean constant studying. I see people learning for years and they don't know much at all, I don't really know why. Doing alittle a day (30min-hour) is better than burning yourself out and not doing it again for awhile.
Just keep at it and learn at a pace you feel comfortable.
I always tell everyone that it takes two years to become proficient in spoken Japanese (after they arrive in Japan). I have heard of remarkable people, however, who have done it in six months.
When I first arrived in Japan, I couldn't speak Japanese for beans. But the people at the school where I taught were helpful. So were all the people at the restaurants and bath houses that I went to. It sounds like a terible challenge, but you will love every minute of it!