[stevenam]

Changed my comment: How do I get Coolreader to recognize duo languagues in 1 book?

For example, I'm using the font Bookerly, and theoritically, any Unicode font should display well. When I read the book in my native language Vietnamese, everything is fine. But when the book contents the Chinese character, all character changed into ???. So, how to fix it?

When I used KOReader, it could display both Vietnamese and Chinese characters seamlessly.
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Nevermind, found a solution myself. So I share here for anybody who did not know it yet.

1. If you also install KOReader on your Kobo, you should be able to find a font called "Noto San CJK" inside the .kobo/koreader/fonts folder. I guess CJK means Chinese, Japanese and Korean. Then this solution should work for Japanese and Korean as well.
2. Copy it to .kobo/vlasovsoft/fonts
3. Open and edit by Notepad++ the file .kobo\vlasovsoft\cr3\data\cr3.ini, line 14

From

crengine.font.fallback.face=DejaVu Sans

To

crengine.font.fallback.face=Noto San CJK SC

4. Save the file
5. Unplug the Kobo
6. ... Profit!!!

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And until Coolreader can handle PDF with reflow, then I can forget about KOReader.