This is eerily well-timed!
My partner and I do something similar for Korean & English (she’s Korean native and is fluent in English and I’m learning Korean). We actually built it out for ourselves and some friends and just released it yesterday[0].
Still working out some kinks, but it sends a question every weekday via email that you’d respond to. It then sends back feedback on vocab & grammar, all with spaced repetition baked in to keep track of words you learn/use as you continue.
It’s currently tailored towards those that can already read and have basics under their belt.