Sorry for making a separate post, but I think I've found the real problem. I tried muxing srt files with mp4 using mp4box/YAMB (both English and non-English subtitles) and when I play the files on iPad the subtitles can't be seen.

Subtitles of videos converted with handbrake can be seen on iPad but it takes a long time to convert. I'm trying to find a faster way. When subtitles are done correctly there is an extra button in the iPad video controls to select subtitles. But videos made my mp4box don't have that.

Is there a way to make subtitles through mp4box work for iPad? Do others have this problem?