Technically yes, but they've been in touch with Duolingo's CTO and it seems like Duolingo is OK with it for now.
https://duome.eu/blog/16.04.2018
> Durung the past few weeks I have been talking to Severin Hacker, the co-founder and CTO of Duolingo. To make a long story short: they do like the idea, but they need to own the domain [context for HN: original domain was duolingo.eu] and they're going to redirect it to duolingo.com so the old forum links will stop working in a couple of weeks from now.