Learn Danish: Hej, hvad skal det være?

Literally, this sentence translates to "what shall it be?" (hvad = what, skal = shall/must, det = it, være = be). It is a set phrase used to ask for someone's order.

Who usually says this, and where would I hear it?

You will hear this phrase constantly in Denmark from cashiers, waiters, bartenders, and bakers. It is the standard, polite way for a service worker to ask what you would like to buy.

The English translation uses "can", so why doesn't the Danish sentence use kan?

Danish uses skal (shall/will) here because it is asking about a future plan or intention (what your order will be). If you used kan, asking "hvad kan det være?", it would sound like a guessing game—meaning "what could it possibly be?" instead of "what do you want to order?"

Why is the verb være used instead of er (is)?

It is because of the helper verb skal. When you use a helper verb like skal (shall), kan (can), or vil (will/want to), the second verb in the sentence must be in its base dictionary form (the infinitive). Therefore, er (am/is/are) turns into its base form, være (to be).