Languages start from your computer's language settings; add or remove any. The first is each person's subtitle language.
When subtitles show: Only when the audio isn't in your language is recommended. A foreign film starts with subtitles in your language; one in your own language doesn't. The other choices are always, only signs and foreign dialogue, or only when chosen while watching.
For: everyone is ticked, the owner included. People you add later in People & folders get the same.