Avix, I sincerely wish you luck in your endeavors. Maybe I am wrong and you did learn more than I think possible. I hope you get to live in Japan.
I studied abroad for eleven months in Argentina, so I can comment specifically about learning Spanish immersion-style. I went in with three weeks of independent study - I knew very basic vocabulary, could conjugate verbs in present tense, and that was pretty much it. In 11 months, I went from almost nothing to at least C1 level Spanish. If I studied vocabulary for a few weeks, I could probably pass the C2 exam. I had less Spanish experience than the OP, and could have worked harder in Argentina. In Spanish, it’s possible, in my opinion, to go from nothing - C2 fluency in a year in an immersion situation. You just must be prepared to work 
Evan, I did the same with German, Italian, and French, and it was a great experience. When I went to live in Japan, I studied in classes for a year, which I found pretty useless once I arrived there; I never mastered it.