The semantic argument behind subtitles being visual but under sound never even occurred to me as something that would come up here. I think this is the best response:
It's an audio setting so it belongs under Audio settings. If the game's settings list Sound, they're foolish and should change that to Audio while they're at it. It does not belong under Display settings just because it appears on screen. That's silly. By that logic all gameplay settings should be under display settings as well. Button prompts appear on screen so there's half of all games' controller settings. Controller sensitivity changes how fast the screen turns on your display so that's a display setting. The AI moves differently on your display when the game is set to hard so that's a display setting too, right? See? Silly.
If games actually included "Language" as an option on menus then that would be best, but since they never do I didn't vote for it. Hopefully one day we get to the point where games ship with a healthy variety of languages supported.