A lot of subtitles from commercial media use a subtitle format that's essentially a bitmap that the video player overlays on top of the video. There are tools to decode this using OCR, but it's not something I'd enable by default.

For text/srt subtitles, translation would probably be easier. There's a plugin for that already if you're okay with online translation services: https://github.com/nopium/vlc-trans-lua