Greasy Fork is available in English.
Creates a button on the top header to show/hide related section
Hides related section on watch page and creates a button on the right of the search box to show/hide it.
Thanks @nightlancerx and @ev-haus for their contribution.
If you want to toggle the related videos with the description, see my other script: https://greasyfork.org/en/scripts/437821-youtube-right-side-description-with-toggle-button
Or just without any toggle button: https://greasyfork.org/en/scripts/438747-youtube-description-on-the-right-side-above-related-videos
UPDATE: A new mod is available, it toggles the description/comments/related vids: https://greasyfork.org/en/scripts/449600-youtube-right-side-description-comments-related-vids-with-toggle-button
