msgfmt "invalid multibyte sequence" error on a Polish text

Using Complete C++ i18n gettext() “hello world” example I changed the locale from "`es_MX`" to "`pl_PL`" and the text from "hello, world!" to "Invalid input. Enter a string at least 20 characters long.". The Polish translation contains several characters that cause "invalid multibyte sequence" errors from msgfmt, "łąźó". The translated text was copied from a web page. I presume utf8 is the problem. If so, what should be used instead? ``` cat >plt.cxx <<EOF // plt.cxx #include <libintl.h> #include <locale.h> #include <iostream> int main (){ setlocale(LC_ALL, ""); bindtextdomain("plt", "."); textdomain( "plt"); std::cout << gettext("Invalid input. Enter a string at least 20 characters long.") << std::endl; } EOF g++ -o plt plt.cxx xgettext --package-name plt --package-version 1.2 --default-domain plt --output plt.pot plt.cxx msginit --no-translator --locale pl_PL --output-file plt_polish.po --input plt.pot sed --in-place plt_polish.po --expression='/#: /,$ s/""/"Nieprawidłowo wprowadzone dane. Wprowadź ciąg przynajmniej 20 znaków."/' mkdir --parents ./pl_PL.utf8/LC_MESSAGES msgfmt --check --verbose --output-file ./pl_PL.utf8/LC_MESSAGES/plt.mo plt_polish.po LANGUAGE=pl_PL.utf8 ./plt ```