To activate subtitles while streaming from your iOS device to your Chromecast, please follow the steps below:

1. Select a video that has subtitles. If there is a CC icon in the description of the movie or TV show, that video will have subtitles available. 
2. Tap on the Chromecast control bar at the bottom of the screen to expand the controls.
3. Tap on the CC button on the bottom left hand corner of the screen.
4. Select English.

Now, English subtitles will be displayed on your screen.