The Extra story and Prologue have been released in Chinese version. But it require a finished record in saved files to access the content. Since the game don't have steam cloud function, some people such as me who played the game two years ago need to finish it again to activate it. Though I recommend that you read it again to recall the memory about the game, you can also use those finished save files to access contents.

https://drive.google.com/file/d/1ClfZuqqY50yB9mfsQ8QNHQfDHsG5lARV/view?usp=sharing

game Properties - LocalFiles tab - browse local files - copy/overwrite content into savedata