@Itai’s answer is great, but I just want to note that it does depend on where you go. Japan is not a country where many people can speak English; not that people don’t like foreign tourists (surely there’s quite a bit of xenophobic sentiment in Japan, but that’s not what I’m talking about here), but many people just can’t express themselves in English at all, an unfortunate consequence of the failure of English language education in the country.
If you just want to go to Tokyo, Osaka, Kyoto, etc., and all the touristy places, then you’ll do fine. You can count on at least one or two people who can speak basic English.
But if you want to go to the countryside, the “off the beaten path” destinations, I’m not sure if you can count on that anymore. While there are some solutions (e.g. Google Translate), you will run into a language barrier. So, when Japan reopens for tourism, I guess you’d want to try the most touristy destinations first. If you decide that you do like the country, you can return and explore the rest of the country, hopefully after having learned a bit of the language (just a bit will suffice).
P.S. Nowadays Wikivoyage is a great alternative for travel guides. It is free and easily accessible, but it still lacks in terms of details and depth. However, for me, it has worked pretty well, at least for Japan.
P.P.S. I have never heard of anyone having trouble with their phones when coming to Japan in the 4G age. Unlike 3G, for which competing standards were a huge pain when people moved across borders, there is virtually only one standard for 4G (LTE), so the vast majority of phones should be usable. Most devices can operate in most LTE bands, so that would hardly be a problem. Voice may not work if you don’t have VoLTE, but (1) most carriers support it nowadays and (2) there’s little chance you’d need to do traditional voice calls. 3G network will be shut down in Japan soon, so you’d better have 4G/LTE service!11