I travelled to Japan last year (April) with a few friends. But from what I saw/experience, you can totally travel Japan solo. Pro of going solo is you can poon and no will ever know....
I would say Japan is one of the easiest/safest non-english speaking countries to travel solo. But, as in any foreign country never let you guard down and pay attention to your surroundings.
I don't speak much Japanese either, the little I know is from watching anime lol. But most Japanese seems to understand a little bit of English and almost all of them have a translator app on their phone. There's also SO MANY tourist and most speak English, so you always ask them for directions.
As someone mentioned before, JR Pass isn't worth it anymore. We didn't get it ourselves and this was before the insane price hikes. Just pay the regular JR line + Shinkansen fee, prices might very similar in the end.
Now here's a tip to navigate the Japan Metro (Tokyo/Osaka). There's a distinction between Metro and Subway in Japanese Metro rails. There's also the local rail system, separate from the JR Lines. If u know the different between these three then you won't get lost. I can't tell u the times we stood on the wrong train platform because we didn't know the different between the Metro and Subway. But if u get lost and need directions the Help Desk Officer speaks fluent English.
Main cities to visits are of course:
-Tokyo and Osaka: Big City feel, lots of nightlife, red light districts etc. Self-Explanatory. Stay in the main city don't stay at nearby city to save money. Not worth it. Commute will eat a lot of your time.
-Kyoto & Nara: Historic capitals, lots of historic sites, temples, castles, museums. Feels like a old Chinese city. Don't stay in Nara, take the train from Osaka or Kyoto. Since its only a 40 min ride and Nara is a small city. Perfect for a day trip. Must do: Feed the Deer + Visit the Temples/Gardens
-For the honsen we when to Hakone. Stay two nights. Trust me on this one.
Visit around March/April time. Weather is perfect, not to cold not to hot. Sakura Season. Try to visit before Golden Week (gets crazy I heard).
Summers in Japan are too hot. But if u wanna climb Fuji. Then u have to go on the Summer since its the only time Fuji is open.
Winter in Japan sucks, too much snow or rain. Stay away.