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’ve used Language Transfer with good success. Don’t even need to sign up, you can just go, click on a course and start streaming lessons. You can also download them locally.
https://www.languagetransfer.org/free-courses-1
The idea is to create rules that help you “transfer” words from the source language into the target language, hence, “Language Transfer”.
For example, going from English to Spanish: Words in English that end in ation, will end in acion in Spanish.
Confirmation -> Confirmación Conversation -> Conversación
Further, words following this rule are “ar” verbs. Confirmation -> Confirmación -> confirmar
Another one is words ending in al, which came to Spanish and English via Arabic… are the same. Just said with an accent.
Normal -> Normal Formal -> Formal
A few rules might get you a few hundred words. And while some words might be more formal than how something is typically said, you should still be understood.
They’re completely donation based, ad-free, and no sign-up required.