Types of English
I'm a native American, so I'm very fluent in English. However, despite that, I have no idea what vegemite is, or what "Dunny Rat" means. Basically I'm saying Duolingo needs to add Australian or British English. Like this comment if you agree!
From what I've observed in the courses I've taken British English is represented and I've even seen an occasional Australian expression. But if we want to see other kinds of English that would be near impossible. Check out these countries that have English as their official language. https://projects.ncsu.edu/grad/handbook/docs/official_language_english.htm
Where to begin? Anguilla to Zimbabwe over 50 and all with equal claim to their language style. Then there are those such as India which has English as one of its official languages. Btw notice the USA is not on the list.
When you say "native American" I'm thinking about tribes of indians in the USA - but I understand what you're saying. I dunno...there are so many variants of US english. I sometimes find it difficult to understand my relatives who live in Tennessee. Some of the terms they use are local slang or words spoken with a heavy accent. However, they can write in a formal US english style when necessary. It would be interesting to observe similarities and differences of both Australian and British english styles. Sounds like a lot of work though.