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 darktable 's PO files:

gettext: src/external/lua-scripts/locale/de_DE/LC_MESSAGES/clear_GPS.po: warning: Charset "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.