Duolingo - Denmark
When can danish people start to study an other language on danish? like english, german or french??
When Duolingo decides on it. Sadly, I wouldn't bet on it from a business perspective. Duolingo is a business and like any business attracting customers is their main goal. What I'm trying to say is that it would seem like an unnecessary task to translate the site and courses into Scandinavian languages as much of the population knows English.
Are you asking if there is any courses teaching from Danish? I.e: German for Danish speakers, Swedish for Danish speakers, etc? If so, there isn't any, unfortunately. Duolingo decides if that'd happen and many Scandinavians already speak English.