Improve KLanguageButton::insertLanguage when no name is passed

Summary:
Move the reading of kf5_entry.desktop from KLanguageButton::loadAllLanguages
to KLanguageButton::insertLanguage that way all the users can benefit
from it

Use QLocale::languageToString if QLocale::nativeLanguageName is empty,
this comes from KSwitchLanguageDialogPrivate::fillApplicationLanguages

Reviewers: apol

Reviewed By: apol

Subscribers: apol, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10444