I have learned Esperanto using Duolingo and I made more mistakes in English (which was the language in which that Esperanto course was tought) than in Esperanto. Mainly because Esperanto is very regular without all those edge cases which English has.
BTW English is my second language, Czech is my first and I have some limited knowledge of German.
BTW English is my second language, Czech is my first and I have some limited knowledge of German.