Neither me nor my significant other are native English speakers, but we have different native languages. I'm comfortable enough with English, but like having English subtitles since I have a hard time with some dialects and occasionally just miss a word or two due to noisy audio. My SO likes having subtitles in their native language.
Being able to have multiple subtitles makes it possible for both of us to get the experience we like, at the cost of a little screen real estate. Well worth it.
If there is no black bar, you could get one of those long narrow LCDs off AliExpress, set up the driver, add it as an X display with the right positioning... and then drag the mpv window across that and the TV. Should work. Those little LCDs are just silly expensive though...