Hello,I just solved an issue with non-latin charachter in user names. I had to set the "free serif" font that is the only one with those charachters included. It was an old documentation page that helped me, when I was redirected to the current doc page this information is missing. I think it should be included as it is not obvious.
Older doc page: https://docs.moodle.org/25/en/Custom_certificate_module#Languages_with_special_character
Current doc page: https://docs.moodle.org/310/en/Custom_certificate_module