Interesting that there is this alternative translation that may be pointing to an informal usage I haven't encountered before. Thanks for pointing to it. But I expect, as the other lesson here on duo states: "Estas son las definiciones que nos da la Real Academia Española."...
(Del b. lat. foranĕus).
- adj. Forastero, extraño.
I agree, it is interesting and I guess one of the tricky things for Duolingo with all the possibilities there are out there for term usage. I first brought this up because I just finished reading its usage as a noun in a book by Gabriel Garcia Marquez. Your link rightly points out that the official usage (RAE) is only as an adjective. I usually stop after checking something in WordReference. I tried to see if it was something available in the Colombian Academy that might clear it up but didn't find much (http://academiacolombianadelalengua.co/diccionarios). Cheers.