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 mumble 's PO files:

podebconf: file debian/po/cs.po not up-to-date
podebconf: file debian/po/da.po not up-to-date
podebconf: file debian/po/es.po not up-to-date
podebconf: file debian/po/eu.po not up-to-date
podebconf: file debian/po/fi.po not up-to-date
podebconf: file debian/po/fr.po not up-to-date
podebconf: file debian/po/gl.po not up-to-date
podebconf: file debian/po/hu.po not up-to-date
podebconf: file debian/po/it.po not up-to-date
podebconf: file debian/po/ja.po not up-to-date
podebconf: file debian/po/nb.po not up-to-date
podebconf: file debian/po/nl.po not up-to-date
podebconf: file debian/po/pt.po not up-to-date
podebconf: file debian/po/pt_BR.po not up-to-date
podebconf: file debian/po/ro.po not up-to-date
podebconf: file debian/po/ru.po not up-to-date
podebconf: file debian/po/sv.po not up-to-date
podebconf: file debian/po/vi.po not up-to-date
      

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.