Someone brought up this issue in the "What is Criticism?" thread. I've been thinking about it and came to some obvious conclusions after a few days.
"Photography" is not a language. it is more akin to pen and paper (or stick and mud or , well you get the idea) . The pen and the paper don't know or care what "language' is being used: japanese, english, it is all grek to the paper and pen.
But some genres, mostly commercial photography, but also some forms of art, documentary, stock, photojournalistic and even erotic photography, are more akin to languages in that there are specific tropes and metaphors and similes that are expected, that are very useful, even expected by the viewer, in communicating the intellectual content & intent of the image.
Think about this for a minute, with reference to "fine art photography": the type of subject; the usual choice of media (large format black & white); the dramatic range of deep blacks through richly atmospheric shadows, detailed mid tones, up to the stratospheres of delicate highlights -- codified as the "Zone System" -- even the type of framing of the subject that is found acceptable- near/far compositions, etc.
Commercial photographs are expected to be boldly graphic.
I'm sure we can look at any set of random photographs and without much difficulty sort them into categories based on some not so random characteristics. sure some images cross over genres, but these are the exceptions, and usually are allowed when the audience is unsure of what to look for. we want these visual clues, we want to know how to respond, how to see , an image.