PlayerXtreme Apple TV app can show wide variety of subtitles. You can show/select subtitles by following the steps below:

Step 1: While the video is playing on Apple TV. Swipe down on touch surface of Apple TV remote and the menu appears.

Step 2: Swipe right on the touch pad to select “subtitles”. A menu displaying all the subtitle options will appear. It takes a few seconds for available matching subtitles (from the video folder) to appear. 

Step 3: You can simply click any subtitle file and it will start displaying on the screen.