It seems to be just an encoding issue, try to fix it following the solutions below:
1) All
email templates located under
/templates/email_*.txt should be saved/stored in UTF-8 encoding. English defaults are in ANSI/ISO. As always, I recommend using Notepad2 for modifying any miniBB file — but in this case, Notepad2 also very properly determines the Encoding and helps to reset it correctly to UTF-8. Here is
the classic how-to proved for years.

2) This option is not set by default, but you can specify it under
setup_options.php ->
$emailCharset = 'UTF-8';
(paste before the closing ?> tag).
These two steps should help in most cases. Let me know if they worked out the issue.