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?
Yomichan (primarily for Japanese but may also work for Chinese and Korean): https://github.com/FooSoft/yomichan
Yomichan itself is no longer maintained, but an actively developed but still beta fork Yomitan exists: https://github.com/themoeway/yomitan
Setup for Japanese: https://aquafina-water-bottle.github.io/jp-mining-note/
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