wrong English translation
the plural for wife is wives, not wifes
"Wifes" is a (very) non-standard plural form of "wife".
Seeing as how it showed up in the German course, it is probably bleeding over from the Pearson courses. Their stuff isn't all that high quality. Unfortunately, because Duolingo didn't put Pearson's contributions into a separate database, this sort of bleed-through is happening. I feel bad for the German course volunteers. It is creating extra work and headaches for them.
Personally, I'm betting it was a typo.
I don't think the people who use "wifes" in everyday conversation are stupid. Language is malleable like that because people made it up to undertake a task in the first place. By now, everything we say was nonstandard at one point in time.
I agree that, outside of a typo, I would be surprised to find it in the course. My knowledge of language is of course, limited. My exposure to it has been largely regional.
In my whole life, I have never heard anyone say "wifes" who did not mean "wives" or, I suppose, "wife's." I'm a native English speaker who has lived in three different American regions, and I would be stunned to find "wifes" used intentionally. Therefore I agree that the inclusion is likely a mistake.