I get that it's a UX thing because most people aren't interested in Swahili subtitles, but it would be nice to have an option to expand the list since I know they have the data.
Subtitles are absolutely limited by region by the studios as part of stopping people VPNing around to access titles that haven't been paid for in that region.
Netflix content that is available in .nl and .br (I'm in .nl, my wife speaks pt-br) only have a couple of subtitles (english, german, sometimes dutch) and the same content in .br has totally different set of subtitles.
appletv+ and disney+ and amazon prime simply offer all available subs. So it's not a UI issue, simply have a look at how they do it.
And then they wonder why people pirate and use opensubtitles/legendas.tv ? Should be really really easy for them to fix.
Prime is worse. I actually watched the movie The Courier with English subtitles on and the subtitles disappeared for all of the Russian dialogue.
But nowadays you can configure additional visible languages yourself on the language settings page (checkboxes under "Programmes & Films Languages") without changing the UI language: https://www.netflix.com/LanguagePreferences
But that seems weird to me - just show the more relevant languages first, then, so they don't need too many button presses.
For example, Netflix DE has subtitles but the Netflix UK won't have the same German subtitles available. Disney+ is doing this better by offering the nearly all subtitles nearly all the time. I can understand that dubbed audio might not be available on other country Netflix' but the subtitles?
Sometimes I am too tired to listen to English and prefer to read subtitles :)