"Can it rain cats and dogs?"
Translation:Kan det regne katter og hunder?
"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"