An app to help couples learn each other's languages (https://learncoupling.com)
My wife is Chinese, and I've been learning Cantonese and Mandarin off-and-on on my own for years. For many reasons, I recognize it's extremely difficult to make it work to acquire a language with the help of a romantic partner. I even had a friend who's wife was a doctorate in French language education but completely failed to use her as a resource.
I've identified some of the pitfalls and am developing a system to get native speaking partners more involved in language learner's journeys in a fun and encouraging way (not as a teacher).
This is my second startup! Went through YC once on my first one (related to VR). Been using YC Startup School this time around.