Duolingo advises that we always use the American English spelling for the default/preferred translation of the sentence, which makes sense for a US based company, and ensures cohesion - even if it's not the sort of cohesion you'd prefer.
All variants of English are of course accepted, even if they're not shown as the default.
In Australia, both storey and floor can be used in reference to a building. I have found it easier to think of etasje as storey rather than floor as this follows my language use of the terms best.
For example if someone lived on the first floor that would be the first level above the entrance, this is also the second storey. Norwegians would consider the person to live 'i den andre etasjen' (on the second storey).