"Hello, who are you?"
Translation:Salve, quis es tu?
It's not better, we are learning pronouns, it's the beginner course, so they include the pronouns.
If you mean that "Salve, quis es?" is more common than the emphatic form, yes, it is. But they won't "fix it" as the purpose is to teach the pronouns.
It's part of the flexibility of the Latin language. We can use the most common word order, and also skip the pronouns, and we can also include the pronouns, and play with the word orders, putting the verb elsewhere than at the end, for instance.
The rule is very simple: use "Salve" if you're talking to one person, and "Salvete" if you're talking to two or more people.
Which form to use doesn't have anything to do with formality in Latin. It's all about singular vs. plural. This word for "hello" is actually a command meaning something like "be well!" and commands have singular and plural forms, just like other verbs.
All pronouns (including interrogative pronouns) are declined, as far as I can recall right now, e.g.
Quis videt? Who is seeing?
Quem videt? Who(m) does (s)he see?
Cuius videt? Whose does (s)he see?
Quid videt? What does (s)he see? (corrected)
Quos videt? Which ones does (s)he see?
I typed my answer instead of using the word bank, and I wrote the plural "Salvete, qui estis?"
My answer was accepted, but I was told that I had a typo, and it should be "Salvete, quis estis?"
This is incorrect; I'm not above typos, but this isn't one. "Quis" is the singular interrogative pronoun. There are also plural interrogative pronouns, which are identical to the relative pronoun. "Qui" is the plural, and it agrees with "estis."