I don't think there is a diploma, but you do get the satisfaction of learning a language and testing it out. That to me, is amazing and a reward. Also, you do get lingots from learning languages on Duolingo. They can be used to purchase skills at the shop, but you already knew this ;) You can brush up on what you learned or give other websites a try. Have fun learning!

If you complete level 25, you should be able to buy for real cash plus a herd of lingots a diploma or shirt or something that indicates your achievement. Make those lingots useful!

I do wish to get a diploma here!