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