use the new form of QTranslator::load() for more flexibility

As the docs explain, the variant of QTranslator::load() taking a
const QLocale& is better because it "uses QLocale::uiLanguages()
and not simply the locale name, which refers to the formatting of
dates and numbers and not necessarily the UI language."  And, using
a default-constructed QLocale permits QLocale::setDefault() to
override the system locale, so for example an application's
main.cpp can do that before constructing a QQmlApplicationEngine.

Task-number: QTBUG-7329
Change-Id: Ia29a4c894087c92b071c0fe484728866f2660fe6
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>