For Unicode aware Delphi versions gnugettext.pas declares a function utf8decode which calls System.UTF8ToWideString. After replacing a call to utf8decode with UTF8ToUnicodeString the problem went away.
Since I don’t want to break backwards compatibility with non Unicode Delphi versions I have now changed gnugettext.utf8decode to call UTF8ToUnicodeString instead of UTF8ToWideString.
I can’t see any problem with this change but I am far from being an expert on Unicode related issues. So, if you find any problem with this change, please comment on the corresponding topic in the international Delphi Praxis forum.