Select the client for which you would like to add the description from the 'Clients' tab from the bar at the left of the screen.
Click on 'Edit' at the top right corner of the screen. A popup will appear.
At 'Descriptions' category choose the language of the description you would like to enter.
After selecting the language, a tab in which you can write the description will appear. Select 'Update client' at the right bottom corner of the popup to save the changes. 