The app could use some spinners, when actions lead to a delay. I clicked on the landing page on the only available purple action button and nothing seemed to happen. I already checked my uBlock Origin, whether it is blocking something, but it does not. Already wanted to reload the page, when finally something visually changed, and the course was loaded. Simply a little spinner/animation would make this way less confusing.

I like, that for keyboard input the special letters are given as buttons, so that I don't need to hunt for those on any US/English keyboard layout.

One thing missing is a way to report mistakes in the learning material. For example I found "Buenos dias" to be translated to "Good morning".