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 nautilus-image-converter 's PO files:

gettext: po/fr.po: warning: Charset "" is not a portable encoding name.
                                                                                   Message conversion to user's charset might not work.
      

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.