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 harden-doc 's PO files:

gettext: de-DE/01_intro.po: can't guess language
gettext: de-DE/02_before-begin.po: can't guess language
gettext: de-DE/03_before-install.po: can't guess language
gettext: de-DE/04_after-install.po: can't guess language
gettext: de-DE/05_services.po: can't guess language
gettext: de-DE/06_automatic.po: can't guess language
gettext: de-DE/07_infrastructure.po: can't guess language
gettext: de-DE/08_sec-tools.po: can't guess language
gettext: de-DE/09_developer.po: can't guess language
gettext: de-DE/10_before-compromise.po: can't guess language
gettext: de-DE/11_after-compromise.po: can't guess language
gettext: de-DE/12_faq.po: faq not a language code
gettext: de-DE/99_appendix.po: can't guess language
gettext: de-DE/Author_Group.po: can't guess language
gettext: de-DE/Book_Info.po: can't guess language
gettext: de-DE/Revision_History.po: can't guess language
gettext: es-ES/01_intro.po: can't guess language
gettext: es-ES/02_before-begin.po: can't guess language
gettext: es-ES/03_before-install.po: can't guess language
gettext: es-ES/04_after-install.po: can't guess language
gettext: es-ES/05_services.po: can't guess language
gettext: es-ES/06_automatic.po: can't guess language
gettext: es-ES/07_infrastructure.po: can't guess language
gettext: es-ES/08_sec-tools.po: can't guess language
gettext: es-ES/09_developer.po: can't guess language
gettext: es-ES/10_before-compromise.po: can't guess language
gettext: es-ES/11_after-compromise.po: can't guess language
gettext: es-ES/12_faq.po: faq not a language code
gettext: es-ES/99_appendix.po: can't guess language
gettext: es-ES/Author_Group.po: can't guess language
gettext: es-ES/Book_Info.po: can't guess language
gettext: es-ES/Revision_History.po: can't guess language
gettext: fr-FR/01_intro.po: can't guess language
gettext: fr-FR/02_before-begin.po: can't guess language
gettext: fr-FR/03_before-install.po: can't guess language
gettext: fr-FR/04_after-install.po: can't guess language
gettext: fr-FR/05_services.po: can't guess language
gettext: fr-FR/06_automatic.po: can't guess language
gettext: fr-FR/07_infrastructure.po: can't guess language
gettext: fr-FR/08_sec-tools.po: can't guess language
gettext: fr-FR/09_developer.po: can't guess language
gettext: fr-FR/10_before-compromise.po: can't guess language
gettext: fr-FR/11_after-compromise.po: can't guess language
gettext: fr-FR/12_faq.po: faq not a language code
gettext: fr-FR/99_appendix.po: can't guess language
gettext: fr-FR/Author_Group.po: can't guess language
gettext: fr-FR/Book_Info.po: can't guess language
gettext: fr-FR/Revision_History.po: can't guess language
gettext: it-IT/01_intro.po: can't guess language
gettext: it-IT/02_before-begin.po: can't guess language
gettext: it-IT/03_before-install.po: can't guess language
gettext: it-IT/04_after-install.po: can't guess language
gettext: it-IT/05_services.po: can't guess language
gettext: it-IT/06_automatic.po: can't guess language
gettext: it-IT/07_infrastructure.po: can't guess language
gettext: it-IT/08_sec-tools.po: can't guess language
gettext: it-IT/09_developer.po: can't guess language
gettext: it-IT/10_before-compromise.po: can't guess language
gettext: it-IT/11_after-compromise.po: can't guess language
gettext: it-IT/12_faq.po: faq not a language code
gettext: it-IT/99_appendix.po: can't guess language
gettext: it-IT/Author_Group.po: can't guess language
gettext: it-IT/Book_Info.po: can't guess language
gettext: it-IT/Revision_History.po: can't guess language
gettext: ja-JP/01_intro.po: can't guess language
gettext: ja-JP/02_before-begin.po: can't guess language
gettext: ja-JP/03_before-install.po: can't guess language
gettext: ja-JP/04_after-install.po: can't guess language
gettext: ja-JP/05_services.po: can't guess language
gettext: ja-JP/06_automatic.po: can't guess language
gettext: ja-JP/07_infrastructure.po: can't guess language
gettext: ja-JP/08_sec-tools.po: can't guess language
gettext: ja-JP/09_developer.po: can't guess language
gettext: ja-JP/10_before-compromise.po: can't guess language
gettext: ja-JP/11_after-compromise.po: can't guess language
gettext: ja-JP/12_faq.po: faq not a language code
gettext: ja-JP/99_appendix.po: can't guess language
gettext: ja-JP/Author_Group.po: can't guess language
gettext: ja-JP/Book_Info.po: can't guess language
gettext: ja-JP/Revision_History.po: can't guess language
gettext: pt-BR/01_intro.po: can't guess language
gettext: pt-BR/02_before-begin.po: can't guess language
gettext: pt-BR/03_before-install.po: can't guess language
gettext: pt-BR/04_after-install.po: can't guess language
gettext: pt-BR/05_services.po: can't guess language
gettext: pt-BR/06_automatic.po: can't guess language
gettext: pt-BR/07_infrastructure.po: can't guess language
gettext: pt-BR/08_sec-tools.po: can't guess language
gettext: pt-BR/09_developer.po: can't guess language
gettext: pt-BR/10_before-compromise.po: can't guess language
gettext: pt-BR/11_after-compromise.po: can't guess language
gettext: pt-BR/12_faq.po: faq not a language code
gettext: pt-BR/99_appendix.po: can't guess language
gettext: pt-BR/Author_Group.po: can't guess language
gettext: pt-BR/Book_Info.po: can't guess language
gettext: pt-BR/Revision_History.po: can't guess language
gettext: ru-RU/01_intro.po: can't guess language
gettext: ru-RU/02_before-begin.po: can't guess language
gettext: ru-RU/03_before-install.po: can't guess language
gettext: ru-RU/04_after-install.po: can't guess language
gettext: ru-RU/05_services.po: can't guess language
gettext: ru-RU/06_automatic.po: can't guess language
gettext: ru-RU/07_infrastructure.po: can't guess language
gettext: ru-RU/08_sec-tools.po: can't guess language
gettext: ru-RU/09_developer.po: can't guess language
gettext: ru-RU/10_before-compromise.po: can't guess language
gettext: ru-RU/11_after-compromise.po: can't guess language
gettext: ru-RU/12_faq.po: faq not a language code
gettext: ru-RU/99_appendix.po: can't guess language
gettext: ru-RU/Author_Group.po: can't guess language
gettext: ru-RU/Book_Info.po: can't guess language
gettext: ru-RU/Revision_History.po: can't guess language
gettext: zh-CN/01_intro.po: can't guess language
gettext: zh-CN/02_before-begin.po: can't guess language
gettext: zh-CN/03_before-install.po: can't guess language
gettext: zh-CN/04_after-install.po: can't guess language
gettext: zh-CN/05_services.po: can't guess language
gettext: zh-CN/06_automatic.po: can't guess language
gettext: zh-CN/07_infrastructure.po: can't guess language
gettext: zh-CN/08_sec-tools.po: can't guess language
gettext: zh-CN/09_developer.po: can't guess language
gettext: zh-CN/10_before-compromise.po: can't guess language
gettext: zh-CN/11_after-compromise.po: can't guess language
gettext: zh-CN/12_faq.po: faq not a language code
gettext: zh-CN/99_appendix.po: can't guess language
gettext: zh-CN/Author_Group.po: can't guess language
gettext: zh-CN/Book_Info.po: can't guess language
gettext: zh-CN/Revision_History.po: can't guess language
      

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.