Is "How is the bank?" correct English?
Yes, it means the same thing as "What is the bank like?" to me, at least if you're asking someone about their experiences with the bank.
It sounds to me more like you're asking how the bank itself is doing :)
Yes, that would be pretty normal colloquial English. Though depending on context it could be asking either about the status of the bank or about the experience of going to the bank.
Why is the translation "How is the bank?" not accepted?
Because it is not how you would say it in English.
A translation option for milyen is 'what kind of', but 'what kind of bank is this?' was not accepted. Don't understand why.
"What kind of bank is this?" translated is "Milyen bank ez?"