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 wesnoth-1.18 's PO files:

gettext: po/wesnoth-anl/es_419.po: can't guess language
gettext: po/wesnoth-anl/racv.po: can't guess language
gettext: po/wesnoth-did/es_419.po: can't guess language
gettext: po/wesnoth-did/racv.po: can't guess language
gettext: po/wesnoth-dm/es_419.po: can't guess language
gettext: po/wesnoth-dm/racv.po: can't guess language
gettext: po/wesnoth-dw/es_419.po: can't guess language
gettext: po/wesnoth-dw/racv.po: can't guess language
gettext: po/wesnoth-editor/es_419.po: can't guess language
gettext: po/wesnoth-editor/racv.po: can't guess language
gettext: po/wesnoth-ei/es_419.po: can't guess language
gettext: po/wesnoth-ei/racv.po: can't guess language
gettext: po/wesnoth-help/es_419.po: can't guess language
gettext: po/wesnoth-help/racv.po: can't guess language
gettext: po/wesnoth-httt/es_419.po: can't guess language
gettext: po/wesnoth-httt/racv.po: can't guess language
gettext: po/wesnoth-l/es_419.po: can't guess language
gettext: po/wesnoth-l/racv.po: can't guess language
gettext: po/wesnoth-lib/es_419.po: can't guess language
gettext: po/wesnoth-lib/racv.po: can't guess language
gettext: po/wesnoth-low/es_419.po: can't guess language
gettext: po/wesnoth-low/racv.po: can't guess language
gettext: po/wesnoth-manpages/es_419.po: can't guess language
gettext: po/wesnoth-manpages/racv.po: can't guess language
gettext: po/wesnoth-manual/es_419.po: can't guess language
gettext: po/wesnoth-manual/racv.po: can't guess language
gettext: po/wesnoth-multiplayer/es_419.po: can't guess language
gettext: po/wesnoth-multiplayer/racv.po: can't guess language
gettext: po/wesnoth-nr/es_419.po: can't guess language
gettext: po/wesnoth-nr/racv.po: can't guess language
gettext: po/wesnoth-sof/es_419.po: can't guess language
gettext: po/wesnoth-sof/racv.po: can't guess language
gettext: po/wesnoth-sota/es_419.po: can't guess language
gettext: po/wesnoth-sota/racv.po: can't guess language
gettext: po/wesnoth-sotbe/es_419.po: can't guess language
gettext: po/wesnoth-sotbe/racv.po: can't guess language
gettext: po/wesnoth-tb/es_419.po: can't guess language
gettext: po/wesnoth-tb/racv.po: can't guess language
gettext: po/wesnoth-thot/es_419.po: can't guess language
gettext: po/wesnoth-thot/racv.po: can't guess language
gettext: po/wesnoth-tools/es_419.po: can't guess language
gettext: po/wesnoth-tools/racv.po: can't guess language
gettext: po/wesnoth-trow/es_419.po: can't guess language
gettext: po/wesnoth-trow/racv.po: can't guess language
gettext: po/wesnoth-tsg/es_419.po: can't guess language
gettext: po/wesnoth-tsg/racv.po: can't guess language
gettext: po/wesnoth-tutorial/es_419.po: can't guess language
gettext: po/wesnoth-tutorial/racv.po: can't guess language
gettext: po/wesnoth-units/es_419.po: can't guess language
gettext: po/wesnoth-units/racv.po: can't guess language
gettext: po/wesnoth-utbs/es_419.po: can't guess language
gettext: po/wesnoth-utbs/racv.po: can't guess language
gettext: po/wesnoth-wc/es_419.po: can't guess language
gettext: po/wesnoth-wc/racv.po: can't guess language
gettext: po/wesnoth-wof/es_419.po: can't guess language
gettext: po/wesnoth-wof/racv.po: can't guess language
gettext: po/wesnoth/es_419.po: can't guess language
gettext: po/wesnoth/racv.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.