"You can say that again!"
Translation:Das kannst du laut sagen!
I thought the same thing, because I’m not a native English speaker and I had never heard the English idiom before... then I read wataya’s explanation here and realized the Italian idiom is exactly the same (I’m Italian): “Puoi dirlo forte!”—“Forte” meaning “loud” (if you’re familiar with reading music sheets, you should know)
If translated literally/as a transliteration " Das kanst du laut sagen" would mean "you can say that loudly" not "can you say that again" which is a question in English, not a statement.
However ""Das kanst du laut sagen" as used here, is an idiom/expression and those very rarely, if ever, can be translated word for word/literally. As an idiom " Das kanst du laut sagen" = "you can say that again'
It's similar to " he bought the farm" in English (US). While it can mean that the person actually bought a farm, as an expression/idiom it means that he died. (I have no idea why, it makes no sense to me, "kicking the bucket" or "pushing up daisies" as expressions for dying make much more sense, but that's just the way it is. Expressions are just something that needs to be learned/memorized in any language.)