I have started with Quick.Cart v6.6. I think file core/orders.php have error - when client received order e-mail last line contains "Wysłane przez program Quick.Cart", but i use English language. It string is not placed with others language strings and i mean - its not better solution. Client cant see and cant change this sentence.
In core/orders.php in the function "sendEmailWithOrderDetails" you will find
// the following phrase must be present in the email's content. See the license - http://opensolution.org/licenses.html $sMailContent .= "\n\n".( LANGUAGE == 'pl' ) ? 'Wysłane przez program Quick.Cart' : 'Sent by the Quick.Cart program';
I think, in case when you use only the english version, just replace the polish text "wyslane przez..." with the english one and the license should not be brokem. You wrote, you use English language. Maybe you did not fully clear/remove the polish language, so in some situations, the mailer takes the polish text into the foot of the e-mail.
Actually, If you didn't notice it's a condition, if language set by the web browser is polish send polish text and if not send english text. Your every client, no matter from where received this polish text ?
OK, the LANGUAGE or $sLang will be determined in config/general.php But if You removed correctly the polish language, then there is no chance, that the "LANGUAGE" will take the "pl" value. Even if lang-in-url is active, and you put sLang=pl into url-address, by correct removed pl-language the file "pl.php" does not exist. And without him no possible to set LANGUAGE to "pl"