Potential Solution

One solution that I could suggest is to make use of the following method:

{{
$this->SetFont($this->get_export_font_name(), '', 16.0 * $this->scale, '', true);}}

for append_comments method defined here : https://github.com/moodle/moodle/blob/master/mod/assign/feedback/editpdf/classes/pdf.php#L254

This ensures that the $CFG->pdfexportfont is also used for adding comments in a different language. 

• As a Teacher ,create an Assignment with File Submissions enabled
• As a student, submit a file for assignment submission. 
• As a Teacher, to go "View All Submissions" and click " Grade" against the students submission.
• As a Teacher, annotate the submission and add chinese character comments using the comments button that gives you yellow post it notes on the submission ( not the Feedback Comments text area box!) . Add 為特區制訂 for example.
• Save Changes. You should see "The changes to the grade and feedback were saved"
• As a Teacher, "View All Submissions" and click on the PDF file generated under the "Annotate PDF" column. 
• Scroll to the last page of the Annotated PDF and you will see that all chinese characters have been replaced with ???