@Alain38 OK: It seems in fact that wat just a problem with the console. By sending the text to the URL (using QUrlQuery for encoding) I have the text with the accents.

@Mark81
Curious. What about:

qDebug() << (QString::fromUtf8(value) == QStringLiteral("citt\u00E0"));

Where value is the JSON value of that particular field (i.e. città).

PS.
What I'm trying to figure out is if this is an output issue, or encoding issue.

@SGaist That is true, thanks!

@Chris-Kawa thanks for the hint. Works now :)

qDebug().noquote() << QString("I have properly opened file %1").arg(fileName);

But please... don't use non-ascii characters in source code. That's just being mean to others.

Oh well. I didn't have a reason for doing that, it was just plain ignorance.
I corrected that, and also one more thing. I eliminate the std::string from

std::string strtmp(attr->value()); oCol.item(i-1)->setText(QString::fromStdString(strtmp));

Standard c++ strings might not cope well with UTF-8 encoding. So I changed it to

QString strtmp(attr->value()); oCol.item(i-1)->setText(strtmp);

These two changes combined have solved it. Thank you very much!

@SGaist Its using Serif. In the terminal there, when he run the app the follow error appears: Qt fontDatabase: cannot find font directory /home/user.../ . Is qt install correctly?
I think the app is searching for a font in the folder of Qt. But if i make the app static, this font dont need be with the app? Well if i need to copy the font files, how to link to my app?