QMimeType comment property should return en_US comment when appropriate translation is empty.

The comment property of QMimeType will use the default language (QLocale().name()) to select the appropriate translation (qt5.11:qmimetype.cpp#L261), Locale comments are load from QMimeDatabase via QMimeProvider and QMimeProvider will parse local shared mime info database (usually at /usr/share/mime/packages/ ) and treat the comment element as the locale value. Accroding to qt5.11 qmimeprovider.cpp#L504 we can see when there isn't a `xml:lang` set to the comment element (meant to be the default locale comment when no matched locale comment available), QMimeProvider will set a `en_US` language code for that locale comment.

When the user's default language is not en_US (for example, zh_CN), and an application haven't provide a comment element with `xml:lang` set to zh_CN but only a default comment element is provided, QMimeType::comment() will return the mime name instead of the provided default comment since it simply can't find the zh_CN comment.

Tested under Qt 5.10 and 5.11. This issue was firstly found with KDE's file manager Dolphin with a file with `application/wps-office.ppt` mimetype and with wps-office package installed.

We can reproduce this issue with a sample shared mimetype database xml like this one (example: /usr/share/mime/packages/Qt-Bugreport-test.xml):

<?xml version='1.0' encoding='utf-8'?>
<mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
<mime-type type="application/qt-bugreport.bug">
<comment>Qt bug report default comment</comment>
<sub-class-of type="text/plain"/>
<glob pattern="*.bug" weight="100"/>
</mime-type>
</mime-info>

Then update mime database and create an empty sample file for test:

sudo update-mime-database /usr/share/mime
touch ~/test.bug

Then when trying to get mime comment from `~/test.bug`, we'll get a"application/qt-bugreport.bug" string (the QMimeType::name() value) instead of the one which should be the default comment. The default comment is set as en_US locale comment (qt5.11 qmimeprovider.cpp#L504) so it should probably return the en_US comment if the user locale comment is empty but the en_US locale comment is not empty.

I'm not sure if it's a bug or it's the designed behavior, so please correct me if I posted with the wrong issue type. Thanks.