This sentence is good to teach you some Romanian speech pattern (very formal, actually) but its choice for the test is bad (I got it during a test, and failed it). That is because even if ro-en translation pose no issue, the reverse is foggy, even for a native Romanian. All the following translations are valid, and there is no rule when to say the one or the other: "Tu saluți pe românește", "Tu saluți în românește", "Tu saluți în română", "Tu spui salut pe românește", "Tu spui salut în românește", "Tu spui hello pe românește" (yes, this actually would be the most used form, think about ”How do you say bună ziua in English?”, the formulation ”How do you say hello in English?” makes no sense), and many other versions using ”a saluta” (to say hello, to greet), ”a spune salut”, ”a zice salut”, ”a da bună ziua”, not counting expressions used not so frequently (or old) like ”a da binețe”, etc.

(@meowagain, yes, ”Tu dai binețe”, ”Tu spui bună ziua”, ”Tu zici bună ziua” are all correct too, and the formulation with ”dai” is used with the same frequency)

why is it "dai" instead of "spui" for "you say"?

Why is it necessary to add "ziua" when, as I've been taught anyway, Romanian people simply use "bună?"