> Once this tool becomes adopted enough societally, the learning of foreign languages is going to become a very niche hobby.

That could a long time or it could be a very different implementation than the one you describe. Half of the world knowing 2 or more languages and that has been growing over time. I don’t see the evidence that technology will soon close the gap of speaking and understanding another language when in comes to communicating with family, music, business, participating in community events, volunteering, or even intimacy.