Unfortunately there is no built-in support for subtitles.
If you want to use subtitles, you will need to implement it. I guess that the best thing to do is just display a TextBlock over the media player. This approach will be most flexible.
Synchronizing the subtitles will involve some kind of a timer. IMO the easier approach will be to refresh the subtilises at a given interval of time, say 100ms. Then you can check the progress of the video and read the correct subtitle for the frame.
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