Student or Learner
Is "settee" a common word in western countries?
Which one is the most common?
'Couch' is too, IMO.
I can't recommend using it in the US unless you are an interior designer or something.
If you say to a normal person "Please have a seat on the settee while I get us something to drink" you'll sound a bit pretentious.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
The word "settee" isn't used much in AmE, other than by interior designers as Barb mentioned. And, in fact, a "settee" in the U.S. usually refers to a piece of furniture smaller than a sofa/couch - it's something more akin to a love seat. I'd read many books published in Britain that made reference to a "settee," and I'd always pictured a small, elegant piece of antique furniture. During my first trip to the UK I discovered that, along with "cooker" meaning a "stove" and "for hire" meaning "for rent", that to those wacky Brits a "settee" was just a plain old couch.