Putting Yourself on a Pedastol: Pepsiman's List of "Educational" Japanese Games
It should be no revelation when I say that if all you're doing when you're learning a language is practicing textbook exercises and using formulaic sentences, you're not making the most of your time nor your potential. Most of the battle of becoming competent with a language and subsequently mastering it comes from actually utilizing it in your day-to-day life, constantly testing yourself and being tested by the world around you. One important way of getting up to snuff and staying up to snuff is to by consuming media in the target language, figuring out how a language is actually used and what that means for those who speak it. Without that effort, theoretical knowledge of how a language operates is nearly worthless, since you can be guaranteed that in even the simplest situations a native speaker won't inherently talk like a textbook.
This list therefore exists for people who understand that and are learning Japanese and want to expand their knowledge through playing video games specifically. Of course, it's always wise to make sure video games aren't your only crutch when it comes to reviewing and refining your Japanese skills in a non-classroom setting, but it probably goes without saying that those who pick video games as their first choice of entertainment in general will likely also find themselves able to better relax and concentrate on a Japanese game than, say, a novel or movie that they might not be as inherently interested in completing. You engage with the language in a game in ways that you can't with other mediums because of the interactivity and there's a certain gratification to be had as a learner when you're successfully able to grasp what the game says or wants you to do and then you complete it without a hitch. If you've come to this list in search of games that will give you that sort of fulfillment, then you've come to the right place. If you're here because you just learned kana and want to know what are good Japanese-only games to flaunt your superior knowledge, then this isn't going to be for you. But since you probably wouldn't have already made it as far as you have in your Japanese if that was your only motivation, that's probably not a huge worry.
As such, the general criteria for a game to make this list is that its writing and (when applicable) voice acting are of a respectable quality. As learning tools, if those things aren't good, then the games aren't going to be worth pursuing as educational tools. This means that a lot of Japanese games with otherwise great gameplay will be omitted since the point isn't to just enjoy yet another game, but to get something out of it as a Japanese learner. To get that across, each game on this list will have some at-a-glance information that will tell you in general terms about the overall merits and difficulty level of the game as a learning tool, with more specific notes afterwards. The difficulty itself will be broken down in terms of how much time you'll have probably need to have spent studying Japanese at a university level to competently handle it, as well as a corresponding JLPT level for those who are learning through self-study. Regardless of that, different games on this list will be good for different aspects of Japanese learning; even at advanced levels, no one game is going to fill in all of the gaps you might still have and that's what the additional details will hopefully cover sufficiently.
Beyond that, the only general disclaimer I have about this list is that there are no guarantees that anything included on this list will have English FAQs readily available online (not a problem if you know your Japanese search terms, though) and that some may also be available in officially localized versions whose "accuracy" as translations I can't guarantee. (Going with an undub version will not inherently solve that problem for a variety of reasons, for the record.) Beyond that, this is an organic list that I hope to continually expand as time goes on, so if a game that you think is good isn't on the list, it's very likely because I just have yet to play it myself. That being said, if you do have suggestions for other games or other such comments, you're free to do so either in the comments section or in a PM.