This is an excellent low-commitment way to pickup and expand vocabulary in a new language if not grammar, and at one's own speed. Highly recommended and nicely executed. I can see a good future for the packs and building in ways to expand learning.
The usability could do with a little improvement as for me I want to know how to pronounce upon discovering each new word (and occasionally thereafter), but (on desktop) the mouse contortion to hit the play button each time don't quite take into account Fitt's law. Be better if it was nearer the target.
Nice clean and elegant design. Although designers rarely consider brand in the context of usage — should be at the bottom of the bubble otherwise my eye has to track over it every time I check a word, thus breaking the learning absorption flow. Maybe a two element UI above and below the original context would work…
The advertising model is really cool, at least if advertisers understand this, as the user is in learning mode so readily open to influence. ;)
