Shortly:
I have a .MP4 movie in wich (some idiot did it, I really cannot understand why) it is included English subtitle, so I can't remove it (like it has been burnt upon the original movie).
I have subtitle in my own language. My question is how can I mask the existing subtitle with a black rectangle, so that the other subtitle, that I'd like to load would be readable?
Is it possible with some Linux media player? I've tried Smplayer and VLC, but could not fulfill the task (or didn't know how to).

Thanks!