What are some games with simple Japanese?
I'm already thinking of playing Pokemon gen 1 considering playing that was how I learned English back then.
Megaman Battle Network games have very simple Japanese, though the majority are translated already. Still, there are 4.5 and the WonderSwan games. I'm going to assume that the rest of Megaman series aren't any more complicated.
I also remember playing a weird action game about chibi robots stylized after Chinese feudal warlords (or, rather, anybody who makes an appearance in Dynasty Warriors). You can tell it was made for kids because it had simple Japanese and all Kanji was transcribed. Special attacks also had complicated Kanji symbols appear in the background, and I still don't know what one of them means.
This might be a bad suggestion, but PSO2. While its language is sort of complicated, the text is very easy to copy-paste and dissect. It also features a variety of stereotypical manners of speech with voice acting, so you can really feel the difference between boku, oré and warewa.
As a bonus, despite the majority of the game having been translated by a fan group, nobody bothered to touch the Episode 3 story mode that concluded with yesterday's update.