I just tried it with German, to see how it works. I'm pleased to see it gives you a placement test first, so you can skip the most basic stuff. My grammar is still good, but there is quite a bit of vocabulary that I never learned, because of the environment I learned in (a high school in Switzerland where I was working). As a result, it put me at level 6, which is really too low. I'm learning words for people, which I already know. But because of the way they check the answers, you really have to be exact, or you get marked wrong. For example, I listened to an audio clip that sounded like "Der ist ein Kind," which is a perfectly normal German sentence. (He/That guy is a kid.) I typed that and it was marked completely wrong because the first word was Er (He), not Der. The start of the audio sounded like a D sound to me. So instead of giving me partial credit, it just takes one of my little hearts.
Then there were problems with the microphone questions. They gave me a ridiculously simple sentence to read (and I have always been complimented on my pronunciation and accent by German speakers). I tried again and again and again, but every time it said "I can't hear you" or "That sounds wrong." I tested my microphone in another program and it worked fine, but the DuoLingo thing just wouldn't accept my answer no matter how slowly or clearly I spoke. So I gave in and clicked "skip" and... it took one of my hearts. The last one. So even though I knew all the answers to everything in this section without even thinking, I still failed it.
So there are definitely some bugs in the system. I finally figured out how to disable the microphone questions, so that won't be a problem in the future, but it will really annoy me when I write something a tiny bit different from what they have as the "right" answer, and even though my answer is also correct, it will be marked wrong. It is, however, a cool way to teach people a language, so I would certainly encourage people to try it (especially English native speakers, who famously refuse to learn second languages - time to change that!).
Maybe in the future I'll try to get some Czech friends together and see about adding a Czech section.