Sometimes when you use characters from different languages in the same book, you may notice that they do not all show up properly in your preview.
If this happens, there are two things you can try.
First, a simple thing you can try is to change the font for your book (not all fonts support all languages).
You can switch fonts by going to your Book Tools and selecting "Writing" and then "Book Theme." If you select "Custom" at the top of the page and scroll down, you select a different font. Then, click the "Update Theme" button at the bottom of the page. The next time you create a new preview, you may see that the characters show up correctly.
A more precise way to do correct the situation, if you are writing in Markua or Leanpub-Flavored Markdown, is to switch fonts back and forth in your book according to these instructions.