I had a quick look around after reading the lead post, and discovered this much. .mp4 does not allow .srt (subtitle) files to be included. There are apps which will grab the .srt file and show the subtitles, if it has the same filename as the .mp4 and is in the same folder. This obviously rules out having multiple .srt files for different languages.
It doesn't look too promising for being able to do it at all on the iPad using the standard Video app, since the favourite container (.mkv), which can contain multiple .srt files) is incompatible with iOS.