Greasy Fork is available in English.
NoteAfter YouTube's new material site design update, this userscript no longer works. However, I found an alternative script that does a perfect job and would recommend it.
Mouseover Popup Image Viewer by kuehlschrank
Ironically, the 'Mouseover' script doesn't work on YouTube's older site, which is why I made Youtube Thumbnail Enlarger, but it now does work with the new site design. I will leave Thumbnail Enlarger up for those who use YouTube's older style:
InfoYouTube stores video previews at a large size then reduces them to fit normal page listings, this script will retrieve and display the original large-sized thumbnail.
Important Note (08/31/17)It seems YouTube has started using a new design which breaks many userscripts, here's how you can easily revert to the classic YouTube layout:
To exit the new YouTube Design layout, you can go to YouTube's homepage, click your channel icon in the top right to pop-up a menu, then click Restore classic YouTube at the bottom of its list.
If you ever want to turn the new design back on, visit https://www.youtube.com/testtube