First, you need to confirm that you've downloaded the DLC and added it to your library.

Then, find the game in your library, right-click on it, and select "Properties".















In the Properties interface, click on "DLC" to check that all your DLC installations are ticked.
























Then click on "Installed Files" and select "Verify Integrity of Game Files".
























Finally, go back to "General", change the language to one different from your current one.
























After I completed the above steps, Steam automatically updated my game. So I'm not sure which step actually worked, but I recommend everyone try them all.

After waiting for the update to download, open the game again and you'll see the "Book" appear.



















Click on it, then select any character and interact with them to get the "The Friends" achievement.



















Return to the main menu, click "Start", and enter the level selection screen.
Click on the cat in the lower right corner to interact and get the "Yokai's Secret" achievement.







































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