Greasy Fork is available in English.
Add speed buttons to any HTML5 <video> element. Comes with a loader for YouTube and Vimeo
< Video Speed Buttonsについてのフィードバック
Sorry work after updated the script:
I has made some tweaks before to add more sites,
so haven't' receive the update (maybe because i had modified the script name?).
Seems to have a problem with Chrome? Tested Youtube and don't see the video speed panel.. On Firefox Quantum an Waterfox Classic that's OK ;