Pretty cool honestly, very nice job. The UX is well made, no distractions, you can consider doing several small sessions during the day to learn during breaks. I love it! I would personally be quite interested in Chinese (in that case, I would strongly recommend putting the accents on latin characters, otherwise users cannot know how to pronounce).
I tried Spanish and Japanese. A tiny recommendation for Japanese: it would be nice to have both kanjis and hiraganas in the same block for the word choices. That way, you can decouple the learning of kanjis from the pure listening.
Great work, really!