Some bigger channels won't allow it, but if they do you can add your own subtitles in whatever language you want here's how...
First, go to the bottom right of the video and click on the settings
Then press Subtitles/CC where you should see this screen
Once you are there press on the Add Subtitles/CC and you should get to a new tab where you see this
It will say to start with entering the language and then you can start adding subtitles to the video,
Not every channel allows this, and it might take time until it fully uploads to the video but this is the method to do so.
When adding the subtitles you will see this in the screen...
And this to publish your contribution
If there is no ability to contribute to the subtitles then it will look like this...
Where it just doesn't show the ability to add subtitles at all