I wouldn't worry about language in Japan, if you go there. Although the older folks didn't learn English the same way the younger folks are, there are so many good accommodations for language, signage and friendly people I never felt stymied in the slightest (obviously, assuming any good traveler will learn and practice a few basics for courtesy and do their best out of both politeness and genuine interest).
I did one solo trip this year, my first since the pandemic and after being a competent and happy solo traveler for many years, and I was a bit disappointed. Maybe it was still the kinks being worked out or whatever, but I always found one of the great things about solo travel being the flexibility to just jump in on anything that was happening in any given locale and enjoy the top sights, adventures, etc. That was not the case for me this spring. I had tons of time and would have jumped in on any day tours, events, anything that was happening that day, the next day, 4 days from now....and everything was either fully booked or needed at least 2 people to happen. I can understand small guiding companies and concessionaires having a hard time taking risks as travel was resuming and only doing activities that were booked well in advance with large groups. The previous years had surely been a financial disaster. But I'll just say that the result for the solo traveler is a real lack of ability for the same sort of spontaneity I used to enjoy when I showed up in a random town in, say, SE Asia or Latin America.
I managed not to catch COVID so far on any of my travels (I've done other non-solo stuff) and I'm grateful I avoided it while restrictions were in place at home and abroad. While in Portugal, we would have had to email the ministry of health to obtain a clearance letter before we could travel home and it sounded like those were hard to come by since they were (surprise, surprise) inundated by foreigners with COVID. But as a vaxxed person, it is now feeling just like any risk of getting sick abroad and the adapatbility you need to deal with it.