The Twitter thread you linked is just somebody pointing out poor performance in Tigrinya. That doesn't mean it performs poorly on all non-English languages. It'

The Twitter thread you linked is just somebody pointing out poor performance in Tigrinya. That doesn't mean it performs poorly on all non-English languages. It's performing poorly on the languages with little representation in the corpus, but those are also the languages that are least likely to be learned on Duolingo. Duolingo users mainly learn languages like English, Spanish, French, German, etc. and the models do just fine with these languages.