Hey I play on a PS4 and game bought in Japan and yes I can set it to Japanese; not sure about one bought outside though. You can set the PS4/game independently meaning my PS4 be in English and the game be in Japanese. On the log in screen there will be a selector on the bottom left for language and just change it to Japanese. No problem to change it back because its the same button.
Also, I just set up my keyboard to type in Japanese and English in game. On your PS4 go to settings > devices > keyboard and just select Japanese.