Hi,
It could either come from the "Charset" setting of the Emails section of the HikaShop configuration, the way your email server handles the sending of emails with special characters, or the way your customers email client display the emails with special characters.
So the question is:
Are you able to reproduce the problem with a gmail email address. If no, then the problem is with the email client.
If yes, then the issue is either with that setting or the email server and you'll have to contact the support of your hosting company in order to see with them what you should put in that setting and if they don't have any problem with the support of special characters for the emails sent by their email server.