While setting email charset is possible in XML, the Designer does not have an UI field to specify it.
It seems logical to me that just putting utf-8 unconditionally in generated XML will be sufficient for almost any purpose.
Currently, the email tasks with non-ASCII character (designed in Designer) arrives as
Content-Type: text/html; charset=us-ascii
Content-Transfer-Encoding: 7bitwith diacritics symbols replaced with '?'.
Should a open an issue for this?