Greasy Fork is available in English.
Automatically enable the subtitles.
How to Install scripts? (Click here)
The script turns the YT subtitles off if it detects hard-coded subtitles in the video.
It only checks if the first 1 minute of the video has subtitles or not. Most trailers/etc have subtitles starting before the first 1 minute of the video, after that 1 minute of video has passed, or after a subtitle is found, the script stops running the checks.
This script uses the Tesseract OCR image recognition library to capture every frame of the first 1 minute of the video, each second.
The 1-second and the 1-minute delay were added to use a bit less CPU power.
Works on mobile browsers as well, but unless you have a powerful mobile device, it is recommended that you use http://greasyfork.icu/en/scripts/479068-youtube-subtitle instead.