A lot of people can get around without speaking Japanese, but I don’t recommend it. You’ll get a much more meaningful and deeper experience out of living here if you can converse with everyone. Randomly stopping and bullshitting with a bored old guy for an hour can be quite fun!
Sadly the developer pay here is pretty dismal, even more so if you come with US expectations. I’ve mentioned this before when it comes up, but the IT sector has organically migrated to a 10am-7pm schedule, so if you factor in commuting and the inevitable overtime then the only time you get to yourself are weekends. Taking multiple days off is frowned upon, although as a foreigner you get a little leniency because you are probably spending two days flying when you visit family.
That all said, it is an extremely convenient country to live in. Great public transportation, great food, friendly people, low crime rates, social health insurance, etc. I really love the living aspect of Japan.
I have a wife and child, and while I could imagine myself living here indefinitely, career wise it’s a dead end - they have trouble fitting you into their social structure. If you are perfectly happy with being a cog in the wheel (or carry your team on your back), and mostly work to live, then it probably isn’t that bad of a choice. I’d definitely encourage anyone closer to 20 than 40 to give it a shot, living abroad is an amazing experience.
That said, I’ll finally be heading back to the US in a couple months once visas clear. The 40-60 age gap is right in front of me, and retirement is taking a stronger grasp on the mind. Much easier to consider options in a higher paying English speaking developer job.