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 xscreensaver 's PO files:

gettext: po/ca.po:80:28: invalid multibyte sequence
po/ca.po:106:21: invalid multibyte sequence
po/ca.po:110:10: invalid multibyte sequence
po/ca.po:110:29: invalid multibyte sequence
po/ca.po:110:38: invalid multibyte sequence
po/ca.po:114:20: invalid multibyte sequence
po/ca.po:119:68: invalid multibyte sequence
po/ca.po:123:35: invalid multibyte sequence
po/ca.po:127:23: invalid multibyte sequence
po/ca.po:143:17: invalid multibyte sequence
po/ca.po:150:12: invalid multibyte sequence
po/ca.po:151:33: invalid multibyte sequence
po/ca.po:159:23: invalid multibyte sequence
po/ca.po:159:36: invalid multibyte sequence
po/ca.po:171:12: invalid multibyte sequence
po/ca.po:175:12: invalid multibyte sequence
po/ca.po:198:16: invalid multibyte sequence
po/ca.po:199:16: invalid multibyte sequence
po/ca.po:211:16: invalid multibyte sequence
po/ca.po:216:18: invalid multibyte sequence
msgfmt: too many errors, aborting
      

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.