Make it possible to provide your own installer translation for the Qt IFW strings to override the one already available

Make it possible to provide your own installer translation for the Qt IFW strings to override the one already available. This will make it easier for people to fix issues found in the translated strings without having to wait for a new release of Qt IFW to have it fixed. This is only a problem with the translations already provided as for other languages it is possible to provide the file without it being overridden.