I queried Google Play about making their films database searchable for captions/subtitles. This is their response.
Thank you very much for contacting me this evening. Currently, closed captioning is available on a portion of our content.
Accessibility of our products is extremely important at Google, so we’re working hard to expand our closed captioning offerings, and make it easier to search for them on Google Play. If you have any further questions, I’d be happy to help.
Elijah The Google Play Support Team