Speaking as a total ignorant from a coding perspective. But I guess that wouldn’t be the hard part, considering that most of Duolinguo is just boxes and text inputs. How difficult it is to create a database of competent linguists with an efficient training who can progressively enhance your understanding of languages?
I think that would go for most learning methods. When you don’t practice a skill you’re always going to get worse with it over time, especially if it is a language.
After a certain point you should be able to retain a language for more than a month - after you’ve attained B2ish level I’ve heard.
What I am saying is, I don’t think people who use duolingo are any better/worse off than most other methods.