Bug while sending mail, sender and topic always in ISO-8859-1

I have got problem with sendimg mail in usermin. I'm from Poland.
In Poland we are using UTF-8 or ISO-8859-2 charset. I don't know how properly set charset in Virtualmin, Webmin and Usermin to don't have problems with characters.
The biggest problem I've got with topic of the outgoing mail.
When I write an e-mail in MS Outlook 2007 and send it to Usermin account I can read all characters.
But when I replay for this letter I will see in Outlook "e" instead of "ę" etc.
I think that even if I set in Usermin ISO-8859-2 charset and the same as Character set for sent mail, E-mails will send with topic and sender name in ISO-8859-1.

How can I change It? My target is to set all charset to UTF-8.

Shoudl I change charset of some config file?
Is it possible to add UTF-8 support in English and Polish Translation of Usermin?

I see that Polish translation is a little bit weakly - I can help with it.