I didn't understand where we can take a 15 min test and earn a certificate. Can anyone help me?
Okay, so I just took the test, and I can tell that it's not just a shortcut to get a certificate. After you have taken the test, you'll be able to see your certificates in the right part of your homepage. It says the following:
Your certificates Spanish: 3.22/5.0 27 minutes ago
Then you can go to a link(http://www.duolingo.com/certs/793zJj), where you can see the full certificate.
You'll need at least 25 lingots to take a 15-minute test. If you have 25 lingots, then you just need to go to the store next to your news-stream to 'buy' it. I haven't taken the test myself, because I'm afraid I'm not good enough yet, but I guess that's all you have to do.
I did a test in French, but I didn't do very well. I'll again when I progress more
It depends on how good you are. It's not that bad, but I'm not really sure how the score is calculated.
It's calculated from the whole tree. If you are, for example, somewhere in the middle and you take the test, you'll probably only know just half of the translations and get a score of 3.5/5. :)