"Can it rain cats and dogs?"
Translation:Kan det regne katter og hunder?
literal translation. Det regner kattunger would be an equivalent. ('It's raining kittens')
I thought this idiom is not used in modern English. Am I wrong? (I am not a native English speaker)
For American English, it's very dependent on your American sub-culture. Which is true for most idioms, as far as I've noticed.
"English" is has lots of varieties so it's difficult to make statements which are valid for all of them. Oxford lists the idiom without making any comment about it being old-fashioned http://www.oxfordlearnersdictionaries.com/definition/english/cat?q=cat so does MW http://www.merriam-webster.com/dictionary/rain%20cats%20and%20dogs
They both just mark it as "informal"
All speakers of Modern English know this expression. It sound rather quaint though. No one I know goes about saying it with any frequency. "It's pissing it down out there!" is more likely.
To be honest, I live in the UK and I've never heard of this idiom in my life.