In order to show subtitles you have three options:
First you need to configure the player controls to use the subtitles plugin:
let config = FPPlayerControlConfigBuilder()
Then you need to configure the player to use subtitles.
If you HLS stream contains subtitles all you need to do is to enable the subtitles plugin for the controls.
You can upload subtitles as VTT files for a video in the Flowplayer Platform UI from the "Files" view. The labels and languages configured will show up in the player.
If you are using FPExternalMedia, you can configure the subtitles by hand:
var subtitles = [FPTrack]()
subtitles.append(FPTrack(lang: "en", src: "https://mycdn.example.com/subs.vtt", label: "English subs", kind: ""))
extMedia = FPExternalMedia(mediaUrl: "https://mycdn.example.com/video.m3u8", adSchedule: nil, metadata: nil, subtitles: subtitles)
Please note that subtitles only work with HLS streams, also when using FPExternalMedia.