As described in the FAQ/Tutorial:
"Native subtitles are missing for the non-native parts of a native audio movie (eg English subs for a non-English part of a mostly English movie, like the Russian spoken in "Hunt for the Red October")"
I have multiple files (mkv and mp4) where the subtitles work fine in VLC and other players but not on the PS3 (neither with PS3 media server, TVersity, windows media server, etc.).
So I can't understand the non-English parts of Borat/X-men First class/Game of Thrones/etc.