Add comment / description to the translations kcm
Summary: Fixes inconsistency with other kcms
Test Plan: comment is displayed in plasma-settings
Reviewers: hein, Plasma, ngraham, bshah
Reviewed By: bshah
Subscribers: bshah, sitter, apol, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D18860