Especially for things like language learning they are amazing, but services like Amazon show like two (English and the local language) out of the 30 something they have available. And I won't even go into the difference of subtitles and dubbing...
For language learning there are some scripts that allow you to hover words and show translations for them. I haven't used them myself with mpv, but I found that kind of tools to be invaluable on a browser when learning a language with a foreign script.
