Konnichiwa friends!
We have great news for all of our fans from the land of the rising sun: a new patch for Fell Seal: Arbiter's Mark has just been released, that adds the support for Japanese language! After you update your game, you will have the option to switch the language and enjoy it with a proper Japanese translation. This applies to the Missions & Monsters DLC as well, so go ahead and grab the whole bundle and start your adventure today!
https://store.steampowered.com/bundle/15253/Fell_Seal_Arbiters_Mark__Missions_and_Monsters_DLC/