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 r-cran-assertive.base 's PO files:

gettext: po/R-assertive.base.pot: warning: Charset "/var/folders/sy/_w4kv6wj37b26k7k52ghxj180000gn/T//RtmpZzZugo/translateme" is not a portable encoding name.
                                                                                               Message conversion to user's charset might not work.
po/R-assertive.base.pot:97: end-of-line within string
po/R-assertive.base.pot:98: end-of-line within string
po/R-assertive.base.pot:99: end-of-line within string
po/R-assertive.base.pot:100: end-of-line within string
msgfmt: found 4 fatal errors
gettext: po/R-ua.po: ua not a language code
      

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.