"I thank you all."
Translation:Ich danke euch.
You could say "Ich danke euch allen" (it is the Dative case, therefore you need the "n" at the end of "alle"). I think the "allen" does not appear in the answer, because Duolingo just uses the "all" in the English sentence to make clear that it expects you to choose the plural form of "you" (euch") instead of the singular form ("dir"). So since there is this difference in German you don't necessarily need the "allen" to indicate that you mean the plural.
Yes, I think it would be much better, rather than putting the English word "all" in, to put the word "plural" in brackets after ("you"). Surely nobody would then think that the word "plural" had to be translated!