Hey there roncmore6,
I apologize about the slow response time, thank you for being patient! Were you able to find the answer to your subtitles question? If not, I'd be happy to help out! Subtitles are based on the app itself, since the app developer is the one who uploads the text files and uses the Chromecast to display it on your screen. So, to answer your question, it really depends on the app itself.
I hope that helps you out, and if you have any other questions or concerns for me, please let me know!