msgfmt "invalid multibyte sequence" error on a Polish text

Question

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

Answer 1

Edit plt_polish.po and change the Content-Type line to "Content-Type: text/plain; charset=UTF-8\n" (Changing the charset from ASCII to UTF-8)