This is a video explaining how to equip your video with subtitles using the tool Aegisub.
You'll also need site webvtt.org
Here is the standard filenames for subtitles in various languages. to be fully compatible with both CDS and Videos, please name the subtitle filename in a standard format, <something>_<ln>.vtt, where <ln> is a two letters ISO language (https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes).
NB! You need to have the script written beforehand!