"The youth is cheerful."
Translation:Młodzież jest wesoła.
After reading the comments here and on another exercise in this lesson, I am still very confused. If młodzież is always a collective noun, then I think the English translation should be "The youth are cheerful" - because we are talking about a group of young people - but maybe I still don't understand.
You are correct that there is a difference in handling collective nouns between American and British English. However, as in all things grammatical, it's a bit more complicated than that.
This blog article explains it well: http://englishasitisbroken.blogspot.com/2007/09/collective-nouns-what-are-they-family.html
Toward the end of the article they discuss collective vs. countable nouns, giving youth as a specific example. This Duolingo exercise is using youth as a collective noun, so it should only take the plural verb in English. By giving "The youth is cheerful" as the translation, you are implying that "the youth" is referring to a single individual.