Now you would think that would be it, but even after enabling those, videos in the YouTube Android TV app did not display subtitles by default.
However after opening a video, pausing it and going to 'more options' and then CC and then manually turning them on, it seems now to be on by default.
No matter which video I open now they appear by default.
I am not sure if this will help since I am on the Shield, but it should be the same YouTube app.
In https://www.youtube.com/account_playback I have 'Always show captions' and 'show automatic captions by speech recognition' enabled.
Now you would think that would be it, but even after enabling those, videos in the YouTube Android TV app did not display subtitles by default.
However after opening a video, pausing it and going to 'more options' and then CC and then manually turning them on, it seems now to be on by default.
No matter which video I open now they appear by default.
Hopefully that helps