Same issue reported here: https://issues.apache.org/ooo/show_bug.cgi?id=111292
It seems like the headless mode does not know how to properly auto-substitute fonts. If you create a docx document with Chinese characters and use a font like Arial Unicode MS (a font that does not exist on my Linux system). Headless mode will churn out a PDF with square blocks.
But, if I open the document in GUI mode, the program knows how to substitute the unknown font for something that exist in my system.
This also applies for Japanese. And I suspect this affects all East Asian languages.
We are also affected by this bug with Libreoffice 3.5.3 on Linux.
duplicate. Just checked with master: works fine.
*** This bug has been marked as a duplicate of bug 51693 ***