Okey, I have seen a lot of languages that you can choose to learn if you're speaking English... Isn't it an idea to try to turn that around? If an English speaker can for example learn Norwegian, Swedish, Dannish, Finnish etc. Why can't they learn English too?
I mean, it would be nice to be able to improve my English skill and improve it despite it being my second language. There are many people that are not very good in English and it would be a fine supplement if people was able to practice and improve English skill. I know I would at least be interested
"If an English speaker can for example learn Norwegian, Swedish, Dannish, Finnish etc. Why can't they learn English too?"
After ten (or more) years of compulsory English lessons, I think that most Scandinavians are beyond the point where Duo's 'introduction to the basics' approach can help them.
I'm not sure about Norway and Denmark, but passing English is required to graduate from school in Sweden.
English is a mandatory class in Norway, yes. You also have to take German or French class in middle school, but that doesn't mean all Norwegians can speak three languages well
No, but you only mentioned English in your OP and more than 90% of its population speaks English. The only reason for the lack of a NO/EN course I can think of is that there aren't any volunteers who see the point in developing one.
Duo only teaches the basics of a language, and I'd think that most Norwegians had a good grasp of those.
There are language courses where there is a reverse tree, in which you can learn English from your target language. You would just have to change the setting of your native language. For example, I finished my French from English tree, and now I'm doing the English from French tree. Some courses don't have a "reverse tree".
Is that what you're talking about?
A very good site I can recommend for English courses is http://www.abaenglish.com/en/english-course/aba-free/ where you can find a free English course where you can try it out. If you sign up for the Premium package (currently about 90 Euros for two years) you get access to all the courses ind there are many video lessons and it even awards Certification from English and American Academies.