You can change the code font here:
https://leanpub.com/YOUR_BOOK/settings/theme
...making sure to replace YOUR_BOOK with your book's unique web address.
To change the code font, scroll down to "Code Font" and select "wqy-microhei-mono" from the dropdown.
Next, scroll down and click the blue "Update Theme" button.
The next time you create a preview, you should see the Japanese characters appear just as you type them in each code block.