If you're planning to use extsub to display more than one language, the software currently requires that SRT files for the same video have the same number of captions with the same start and end times, regardless of language, with a maximum of two lines per caption. (e.g. The demo video has 88 caption.) This limitation will be fixed in future iterations.
I'm also working on accommodating other screens with better aesthetic capabilities. More soon on that!