Fonts for languages other than English are not set.

[IknowQT]

    QTextCodec::setCodecForLocale(QTextCodec::codecForName("eucKR"));

int id = QFontDatabase::addApplicationFont(":/MainWidget/Ubuntu-R.ttf");
    QString family = QFontDatabase::applicationFontFamilies(id).at(0);
    QFont SerifFont(family);
    SerifFont.setStyleStrategy(QFont::PreferAntialias);
    a.setFont(SerifFont);

I downloaded the ubuntu font and applied it. English works well, but Korean doesn't. The font supports Korean. Is it a problem with other settings? The same goes for other fonts in Windows.

Is there a way to apply the same font to all languages?