When setting the language from the default to lets say dutch the E-mail you recieve on subscribing and unsubscribing arre still (not all) in english..

Propably dutch translation is not finished. You have a chance to change it - just find .../messages/nl/LC_MESSAGES/mailman.po file, and translate needed messages.

Before you start I advice to contact with Danny Terweij danny@terweij.nl who is champion of dutch translation.

BArtek