So, I do not live in Japan, but I've visited multiple times now. I don't speak Japanese more than a few words here and there and mainly get by usually with Google Translate.
I've had a not-too-difficult time making friends (who I even stay in touch with from here in the US) by finding people there who participate in the same hobbies (car culture and baseball, specifically). Generally, most people I've met in those communities are very friendly. Not everyone enjoys the Google Translate back and forth but many still do.
Anyway, like I said before, I don't live there so it may be different as a long-term resident. But I just feel like based on my experience so far, the "way in" may just be a matter of finding people with similar interests, much like I would here in the US.