Subtitles are definitely in the works, and we're eventually hoping to source as many translations as possible. We even recently added our first spanish documentary! (https://app.curiositystream.com/collection/221/) Hoping to have the previews live soon.
Hi, I like the concept and will definitely give it a try! It seems like you have a lot of good science content which is right up my alley.
My wife is a Spanish ↔ English translation consultant who has a lot of experience with subtitles. Feel free to ping me if you're interested in chatting with her--email's in my profile.