Use registered categories in the desktop file.

Summary:
The desktop file spec has a 'Languages' additional category that can be used
for artikulate.
(https://standards.freedesktop.org/menu-spec/latest/apas02.html)

Reviewers: apol

Reviewed By: apol

Subscribers: kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D24551