When QMimeProvider parses the shared mime database xml files,
it will read the <comment> element for mime comment and treat the
`xml:lang` attribute as locale language string. When no `xml:lang`
attr is provided, QMimeProvider will read the value and treat it as
a en_US locale string as the default key.
When we call QMimeType::comment(), it will try to get the locale
comment string with the default language (QLocale().name()), once
it can't find a matched result, it should return the default key
(which QMimeProvider set it as en_US locale before) as fallback.
Task-number: QTBUG-71314
Change-Id: I444f8159d6f19dfef6338cd79312f608d8f13394
Reviewed-by: David Faure <david.faure@kdab.com>
e3c84b6da1