You can check if a PO file is valid with the following command:

        msgfmt -c -o /dev/null <po file>
      

The following errors were found in mailman3 's PO files:

gettext: src/mailman/messages/zh_Hans/LC_MESSAGES/mailman.po: can't guess language
gettext: src/mailman/messages/zh_Hant/LC_MESSAGES/mailman.po: can't guess language
gettext: src/mailman/testing/mailman-xx.po: xx not a language code
gettext: src/mailman/testing/mailman-yy.po: yy not a language code
      

Please ask the translator (identified by the Last-Translator field in the PO file), the language team (identified by the Language-Team field) or debian-i18n for a fix.