change return type of QLocale::languageToString from QString to QLatin1String.

suppose code:

static inline QString locname(const QLocale::Language i)
{
    return QString::fromLocal8Bit(dgettext("iso_639", QLocale::languageToString(i).toLatin1().constData()));
}

it convert QLatin1String inside languageToString() into QString, and then (in my code) it conerts back to QLatin1String. Why not remove internal conversion fromQLatin1String to QString ?

Also, if returned string is always non-internationalized string, it is more clear to set return value QLatin1String.