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 unknown-horizons 's PO files:

gettext: po/scenarios/af/The_Unknown.po: can't guess language
gettext: po/scenarios/af/tutorial.po: can't guess language
gettext: po/scenarios/ar/The_Unknown.po: can't guess language
gettext: po/scenarios/ar/tutorial.po: can't guess language
gettext: po/scenarios/bg/The_Unknown.po: can't guess language
gettext: po/scenarios/bg/tutorial.po: can't guess language
gettext: po/scenarios/ca/The_Unknown.po: can't guess language
gettext: po/scenarios/ca/tutorial.po: can't guess language
gettext: po/scenarios/ca@valencia/The_Unknown.po: can't guess language
gettext: po/scenarios/ca@valencia/tutorial.po: can't guess language
gettext: po/scenarios/cs/The_Unknown.po: can't guess language
gettext: po/scenarios/cs/tutorial.po: can't guess language
gettext: po/scenarios/da/The_Unknown.po: can't guess language
gettext: po/scenarios/da/tutorial.po: can't guess language
gettext: po/scenarios/de/The_Unknown.po: can't guess language
gettext: po/scenarios/de/tutorial.po: can't guess language
gettext: po/scenarios/el/The_Unknown.po: can't guess language
gettext: po/scenarios/el/tutorial.po: can't guess language
gettext: po/scenarios/eo/The_Unknown.po: can't guess language
gettext: po/scenarios/eo/tutorial.po: can't guess language
gettext: po/scenarios/es/The_Unknown.po: can't guess language
gettext: po/scenarios/es/tutorial.po: can't guess language
gettext: po/scenarios/et/The_Unknown.po: can't guess language
gettext: po/scenarios/et/tutorial.po: can't guess language
gettext: po/scenarios/fi/The_Unknown.po: can't guess language
gettext: po/scenarios/fi/tutorial.po: can't guess language
gettext: po/scenarios/fr/The_Unknown.po: can't guess language
gettext: po/scenarios/fr/tutorial.po: can't guess language
gettext: po/scenarios/frp/The_Unknown.po: can't guess language
gettext: po/scenarios/frp/tutorial.po: can't guess language
gettext: po/scenarios/ga/The_Unknown.po: can't guess language
gettext: po/scenarios/ga/tutorial.po: can't guess language
gettext: po/scenarios/gl/The_Unknown.po: can't guess language
gettext: po/scenarios/gl/tutorial.po: can't guess language
gettext: po/scenarios/hi/The_Unknown.po: can't guess language
gettext: po/scenarios/hi/tutorial.po: can't guess language
gettext: po/scenarios/hr/The_Unknown.po: can't guess language
gettext: po/scenarios/hr/tutorial.po: can't guess language
gettext: po/scenarios/hu/The_Unknown.po: can't guess language
gettext: po/scenarios/hu/tutorial.po: can't guess language
gettext: po/scenarios/id/The_Unknown.po: can't guess language
gettext: po/scenarios/id/tutorial.po: can't guess language
gettext: po/scenarios/it/The_Unknown.po: can't guess language
gettext: po/scenarios/it/tutorial.po: can't guess language
gettext: po/scenarios/ja/The_Unknown.po: can't guess language
gettext: po/scenarios/ja/tutorial.po: can't guess language
gettext: po/scenarios/ko/The_Unknown.po: can't guess language
gettext: po/scenarios/ko/tutorial.po: can't guess language
gettext: po/scenarios/lt/The_Unknown.po: can't guess language
gettext: po/scenarios/lt/tutorial.po: can't guess language
gettext: po/scenarios/lv/The_Unknown.po: can't guess language
gettext: po/scenarios/lv/tutorial.po: can't guess language
gettext: po/scenarios/ml/The_Unknown.po: can't guess language
gettext: po/scenarios/ml/tutorial.po: can't guess language
gettext: po/scenarios/nb/The_Unknown.po: can't guess language
gettext: po/scenarios/nb/tutorial.po: can't guess language
gettext: po/scenarios/nl/The_Unknown.po: can't guess language
gettext: po/scenarios/nl/tutorial.po: can't guess language
gettext: po/scenarios/pl/The_Unknown.po: can't guess language
gettext: po/scenarios/pl/tutorial.po: can't guess language
gettext: po/scenarios/pt/The_Unknown.po: can't guess language
gettext: po/scenarios/pt/tutorial.po: can't guess language
gettext: po/scenarios/pt_BR/The_Unknown.po: can't guess language
gettext: po/scenarios/pt_BR/tutorial.po: can't guess language
gettext: po/scenarios/ro/The_Unknown.po: can't guess language
gettext: po/scenarios/ro/tutorial.po: can't guess language
gettext: po/scenarios/ru/The_Unknown.po: can't guess language
gettext: po/scenarios/ru/tutorial.po: can't guess language
gettext: po/scenarios/sk/The_Unknown.po: can't guess language
gettext: po/scenarios/sk/tutorial.po: can't guess language
gettext: po/scenarios/sl/The_Unknown.po: can't guess language
gettext: po/scenarios/sl/tutorial.po: can't guess language
gettext: po/scenarios/sr/The_Unknown.po: can't guess language
gettext: po/scenarios/sr/tutorial.po: can't guess language
gettext: po/scenarios/sv/The_Unknown.po: can't guess language
gettext: po/scenarios/sv/tutorial.po: can't guess language
gettext: po/scenarios/th/The_Unknown.po: can't guess language
gettext: po/scenarios/th/tutorial.po: can't guess language
gettext: po/scenarios/tr/The_Unknown.po: can't guess language
gettext: po/scenarios/tr/tutorial.po: can't guess language
gettext: po/scenarios/uk/The_Unknown.po: can't guess language
gettext: po/scenarios/uk/tutorial.po: can't guess language
gettext: po/scenarios/vi/The_Unknown.po: can't guess language
gettext: po/scenarios/vi/tutorial.po: can't guess language
gettext: po/scenarios/zh_CN/The_Unknown.po: can't guess language
gettext: po/scenarios/zh_CN/tutorial.po: can't guess language
gettext: po/scenarios/zh_TW/The_Unknown.po: can't guess language
gettext: po/scenarios/zh_TW/tutorial.po: can't guess language
gettext: po/scenarios/zu/The_Unknown.po: can't guess language
gettext: po/scenarios/zu/tutorial.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.