That’s definitely more of a tricky one.
It might work in some way with text-based subtitles if you have the forced subtitles for each language you want to support…
- “English” forced subtitle track will display when spoken language is Spanish or Russian
- “Spanish” forced subtitle track will display when spoken language is English or Russian
Such a feature would be limited to 2 languages (1 atop the movie, 1 below) – and it’ll be a pain for non-letterboxed content as subtitles will eat 2x the space from your content. Image based subtitles from DVD/Blu-Rays won’t work as they can place their subtitles all over the screen (same for formatted subtitles like ASS).
While I see where you’re coming from, this seems more of a far stretch to be implemented (e.g. because of the selective use cases it can be applied to; but I’m not the judge of that 
)
If you’re working with SRTs you could use some available SRT Merger app to achieve what you’re looking for (there’s even some web apps doing this – just google merging SRT subtitles).
There’s already an existing feature suggestion for placing subtitles outside the video for letterboxed content (e.g. 21:9 content on 16:9 display)
