I suggest to first make sure you set the right charset in your AS
Setting the JVM properties file.encoding or mail.mime.charset DO affect the charset used for the [text] content from the mail template, but neither seem to have any effect on the [html] content.
I also tried setting these properties in the jbpm.mail.properties file (which is where I'd expect mail.mime.charset to have some effect) but they had no effect on [text] or [html] at all.
what is the character setting of the html mail when it arrives?
Content-Type: text/html; charset=us-ascii