Is there a certificate mailed upon completion of a language?
I don't know about anyone else but I would kinda like to have something tangible that I can hold that says I did it! I know thats what actual fluency tests are for I guess but maybe it would be a good thing for duolingo to think about! ;)
When you finish a skill tree, you will get a golden owl at the bottom of your tree. If you click on this, a certificate will pop up, it looks like this:
You could take a screenshot of the certificate and then print it on some fancy paper to make it look professional. You can edit it using Photoshop to remove any unwanted things like the 'share' and 'tweet' buttons. This is what I did.
I hope this helps! If you have any more questions feel free to ask me:)