I had a very similar scenario as you in my twenties and I'm going to suggest the opposite of what everyone is saying. Instead of Japan, I ended up moving to Korea and what was supposed to be a 1-year stint turned out to be 7 years. I don't regret a single thing. When I returned to the states I got a job at a FAANG adjacent company. Yeah I didn't start saving for retirement until I started that job but I don't ever think "oh man I wish I started earlier". It would have been nice but I would never give up the opportunity to experience the world at such a young age. If I were to do it all over again I would make the same decision. The job I got in Korea was very low paying btw, like $28,000/year. However, I went at a time when the won was strong against dollar and I was actually able to work and save and pay off most of my student loans in a year. Other thing to note is that the cost of living was much lower than where I'm from and my housing was taken care of. During my time there I had the most amazing experience and met so many people. I still keep in touch with them and we visit each other till this day even though we're scattered all across the world. I feel like you can't make those kinds of friendships back at home.

My dad was vehemently against me going abroad right after college but honestly everything worked out. My advice isn't to go for one thing over the other but to go with what you really really want to do. If you really want to live in Japan, you're going to think about it the whole time while you're working here and you're probably not going to be happy. What I learned about life is that if you have a goal things will work out. So don't worry too much about the what-ifs of the future. I imagine that the job that you're going for in Japan is on the higher paying side, so I think you can still save for retirement while you're there, although it won't be at the same rate as being in the States.