You can get by without speaking Japanese as a normal tourist, but if you try to live in a van and get hassled by the police, lacking Japanese language skills will not go well for you.
Related to this, a good idea when traveling internationally is to carry contact information for a local embassy or diplomatic branch of your home country.
One thing a traveler could do for some peace of mind is notify an embassy of the trip ahead of time, such that in a police hassling scenario someone on scene can call and get official confirmation in Japanese of who you are and what you're doing, act as an interpreter, etc.
To note, this is not about getting shot in the night or forcefully locked for weeks without recourse.
Just spending hours trying to convince local cops that you’re not a drug dealer, having them look at papers they don’t understand, and getting more lecturing on how you wasted their time. And that every few days as you move to new places.
Of course shit can happen, but it will be the exception more than the rule.
