Strange.
I did a full clean build many times.
Fresh ZIP file from github etc.
The main.cpp idea was causing an error during the build.
Code:
W:\Develop\Sigil\Sigil-master\src\main.cpp(146): error C2761: "void QLocale::setDefault(const QLocale &)": Ponowna deklaracja składowej nie jest dozwolona
[ang. member function redeclaration not allowed]
Anyway setDefault for QLocale:
If this function is not called, the system's locale is used.
In the attached gif I show the problem.