Actually, the Greek translation for Mailman is new. It was not in even 2.1.14. It is in 2.1.15rc1 released last month and will be in 2.1.15 which will be released next week.
If you want Mailman's admin and admindb pages in English, the only way to include non-ascii in the nonmember_rejection_notice and have it render properly in the notice sent to the user is to change Mailman's character set for English to utf-8 as described in https://bugs.launchpad.net/mailman/+bug/1010259/comments/1. This is because even though there are ways of putting non-ascii, e.g. utf-8, characters in nonmember_rejection_notice, the actual reject email sent to the user will say the charset of the message is that of the list's preferred language.
** Changed in: mailman Importance: Undecided => Wishlist
** Changed in: mailman Status: Incomplete => Triaged
** Changed in: mailman Assignee: Mark Sapiro (msapiro) => (unassigned)