As mentioned in this discussion, personal messages containing international accented characters (with diacritics, UTF-8 encoded) do NOT carry over correctly to the forwarded email copies of these messages.
Moodle's internal messages interface displays such messages correctly; however, forwarded email copies look corrupted (unencoded?).
I have tested these languages: Polish and French. I have added screenshots of the test examples tested.
I have tested directly Moodle versions: 3.9.8 and 3.11.1 (the MoodleCloud site I administer).
The bug does not affect Moodle 3.6.10.
Interestingly, the problems appears to ONLY concern the typed contents of personal messages - NOT notifications about Insights or forwards from subscribed forums. Also the email test message sends correctly. So this file outputs what it should /admin/testoutgoingmailconf.php , and the Polish language string "testoutgoingmailconf_message" is rendered correctly in the forwarded email of the test message.