To be frank I've never done subtitles, but I've had issues with DVDSP from time-to-time... usually my fault. I also watch a lot of films with English subtitles and know that individual subtitles sometimes "disappear" in a flash.
Have you played the build file in DVD Player to determine whether the authoring or the playback hardware is at fault? If the build file plays OK in DVD Player, then the problem may be excessively high bitrates from all the subtitles. That might explain the varied behavior on different players.
It may be that the Build compiler is smart enough to see that all the subtitle tracks were the same when you were using English placeholders. This would explain the change in behavior once the languages changed. It's also possible that font choice or a large numbers of diacritical marks may play a role.
It is also possible that your DVDSP project is corrupt. I had an experience with that and nothing would fix the odd behavior. If you re-create the project, you may want to do it in phases, progressively adding languages so you can easily "back up" if things go bad again.
hope this helps