Well I've tried Norwegian a little, and have completed the Esperanto one (both are only a few months old). Here's some differences of Esperanto and German (German is also one of the original oldies). 1) At the top of the page, there is no "Words" or "Immersion" feature for Esperanto. 2) No Fluency Level Badge meter thingy (not that it really matters because it's not really accurate) for Esperanto 3) None of the following optional skills in shop: Idioms, Flirting, Christmas for Esperanto 4) German has a refined and expanded tree I believe. At the very least it has probably twice the amount of lessons as Esperanto. Some things that are also probably true is that German has less bugs, less errors, and more comments per post, but I can't confirm that. You can use the newer languages on Duolingo without much difference.