silaswulff - perhaps you'll find something valuable in this topic: http://opensolution.org/Quick.Cart/forum/?p=readTopic&nr=5422&page=999 A bit down you will find treewood's and Makaron's comments.
yes, I can confirm you have an encoding problem in the libraries/Trash.php. Links like for example http://www.campingxperten.dk/?campingm%C3%B8bler,15 show that change2Latin() function is not working properly.
Please try to add the custom characters of your language to the change2Latin() function to see if it can be mitigated easily or not. Don't forget to make sure save the whole Trash.tpl in UTF-8 format (UTF8 without BOM in Notepad++) in UNICODE compatible editor (Notepad++ is very good).
btw the correct behavior of basic email in QC is to translate these special characters to ASCII characters, for example: Campingmøbler should look like Campingmobler
yes, just add 'ś' to the upper line and its equivalent in ascii ('s') to the lower line. And also delete the characters you don't need in your language (those Polish chars) because the more characters, the slower the webpage is generating.