I found that when I want to see the result of Survey activity graph result in my language, It is not shows characters of graph in proper way and all of them is like square box like as screenshots! Then i tested in some other language such as Arabic and Farsi (RTL langs) and Korean, ...

So Why is such issue happen? is it related to default font decoding/encoding  that used? or something else?   
I tested on https://qa.moodledemo.net/mod/survey/view.php?id=45 as sample