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 mtpaint 's PO files:

gettext: po/fr.po:1220: 'msgid' and 'msgstr' entries do not both end with '\n'
po/fr.po:1459: 'msgid' and 'msgstr' entries do not both end with '\n'
msgfmt: found 2 fatal errors
      

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.