Quite fancy playing this game but don’t want the feeling of ‘you should be practising Japanese’ whilst gaming, so is it possible to switch it to Japanese language?
I imagine that it’s the sort of thing that one thinks is quite easy but in practice is difficult.
I guess the fact it’s my son’s ps5 is not especially relevant… unless it means changing the whole setting to Japanese… he would not like that.
From my experience with a US region PS4, changing the language settings only affects the system interface. I would imagine that carried over to the PS5. Changing the language on the Switch does change the game text (for some games) so I was hoping that would work for PS games. It did not. 
As far as I can tell, the only way to play games in Japanese on non-Japanese systems is to either obtain a Japanese copy of the game or to create another PS account and set the region to Japan and buy games from the PS store. I haven’t actually tried it myself, so I don’t know how annoying the process is.
I hoped that helped at all!