Don't forget that their porn is blurry. Such a travesty.

I’ve been told that if you can get a transfer there you need to understand that you’ll only ever hang out with other westerners because the culture is impenetrable.

I've lived in Japan and that's completely untrue. Sure lots of westerners have this experience, but it's usually for one of three reasons:

1. Language barrier: if you don't speak Japanese, obviously you're not going to fully integrate. However English is a required subject in schools, and while many Japanese are hesitant to speak it (mostly out of embarrassment over mistakes) there are plenty who are eager to utilize what they've learned. And this doesn't even include Japanese who've lived abroad and are properly fluent and comfortable in the language (there aren't many, but they certainly exist). Learning even a little Japanese and appearing to make an effort will also open a lot of doors.

2. Self-segregation: It's easy to fall into this one, especially because of point 1. But if you make an effort to make friends with people who are Japanese, they'll introduce you to their friend group. This works best if your co-workers are mixed westerners and English-speaking Japanese, but with some searching you should be able to find mixed socialization opportunities outside of work too. Plus there are non-Western communities in Japan too: folks from other Asian or Latin American countries who also feel like outsiders. Of the foreigner friends I had in Japan, almost none were American (and almost none were native English speakers, although they were all fluent or close to it). Taking Japanese language classes is a great way to meet these folks, but each school is going to be very different in their demographic populations so be choosey and avoid any that seem too homogenous (eg all Chinese students).

3. Expectations: Japanese social culture is very different. It's easy to make acquaintances and hard to make friends. And yes, even if you speak Japanese fluently as a foreigner you will find it more difficult to make friends with the average Japanese person. However you have two strengths you need to utilize: curiosity and otherness. As long as you appear approachable, strangers will sporadically come up to you and talk with you out of curiosity. Sometimes the socialization ends there, and that's fine. But look for the outsiders: Japanese people who don't feel like they fit into Japanese society. I found that there were a good number of English-speaking Japanese who wanted to be my friend because they struggled to make friends with other Japanese. Some of them were socially awkward or autistic, some had lived abroad, and some seemed totally normal. The good news is you probably have a lot in common, even if it's superficial like enjoying the same Western culture (TV shows, etc; think reverse-weeabo), but it's enough for the foundation of a friendship. However you'll need to accept that you're now a member of a misfits social group, because you're yourself a stranger in a strange land!

So in conclusion, it's very easy to fall into the only-socializing-with-Westerners trap, but it's certainly not inevitable. It takes conscious effort to avoid, but is completely feasible, especially for someone who has interest in Japan/Japanese culture/Japanese language.