https://www.nexusmods.com/okamihd/mods/4
If you use French, German or Japanese, add _ and your languages first letter (F, G or J respectively).
Move the files to steamapps\common\Okami\data_pc\movie and overwrite the files if asked.
Source Thread:
https://steamcommunity.com/app/587620/discussions/0/2549465882934705888/
Video Tutorial:
https://www.youtube.com/watch?v=KrikQWFfpXs