Why not "This is THE living room"? If this isn't valid, how would it be written?
adding (the) to the sentence would make it as: هذا الصالون haathaa ass-ssálún
and by the way, the word for "living room" here is not Arabic... they should have used (غرفة المعيشة) for living room instead.
Strangely enough, searching Google for images gives a totally different style of living rooms according to the term you use.
I mean, depending on whether you use صالون or غرفة المعيشة the results are totally different. With غرفة المعيشة, the images show modern furniture, whereas with صالون, you get classic furniture.
Well, I'm not sure about the "style" of the furniture, but for sure you can't completely depend on google image results.
The word صالون is probably derived from French (Salon) or something and to my knowledge it is mostly used now for the barber shops (and ladies' hair styling shops). But putting aside dialects, and focusing on MSA, the living room is غرفة المعيشة and you can check it out on Google Translate.
Just occurred to me that some people also call the Guest Room as "صالون" - so we really have to stick to the MSA meanings. Dialects come later