Neat! I had a very similar idea recently: https://seamlang.app/ (the sign up button doesn't work yet)

The main difference seems to be that I start with text in the foreign language, and then translate the difficult vocabulary back to the known language (English). That way you always ensure you have the correct grammar of your goal language even if you don't know most of the vocabulary yet. This can be a bit confusing at first because you have mostly English text with Spanish word order, but just trying it a bit it works pretty well. It also makes the difficulty an easier problem because the grammer stays the same.

I haven't gotten around to finishing it yet, especially judging which vocabulary to translate and ensuring each translated word still makes sense in context isn't easy.