Translation:It is not the eleven Brazilian boys who are good, but the eleven Germans.
This is weird English. A normal person would say this as "It is not the eleven brazilian boy who are good, but rather the eleven german boys." You can omitt the rather or replace the german boys with just germans, but that's about it. The suggested solution isn't technically wrong, but it isn't something I would ever expect to see used.
You are quite right. As the noun is "boys" it should be "German" not "Germans"...
Otherwise it sounds like there are 11 boys who are Brazilian another group of 11 German people (regardless of age and gender).
Saying "German" makes sense and clarifies that it is the 11 German boys rather than some other group of 11 Germans.
"It is not the eleven Brazilian boys are good, but the eleven German." Is right and natural. Although "German boys" would be the most natural. In fact, in English it would be more natural to omit the "eleven" than the "boys" part.
Sigh... For this Hungarian course, I wish Duo would use real sentences from media and literature rather than making up stuff and then translating it badly.