If you are the video owner, you can make captions appear by default by
adding this tag to your video: yt:cc=on. It won't stop people from
turning them off by clicking the CC button, but it will default
captions to 'on' for most users.
When you embed a video on another site, you can make it so that
captions are always shown on the embedded video. To enable captions on
a video you'd like to embed, just add &cc_load_policy=1 to the video's
You can also choose a caption language for the embedded
video. To specify the caption language for the video that you'd like
to embed, just add &hl=fr&cc_lang_pref=fr&cc_load_policy=1 to the
video's embed code. Here, hl sets the language for the player menu,
cc_lang_pref sets the language for the captions shown in the video,
and cc_load_policy=1 makes captions on by default. fr represents the
ISO 639 language code for French. You can look up 2-letter language
codes in the ISO 639-1 standard.