Theory

[icycalm]

Because it sure as hell doesn't exist in mine, or in Japanese for that matter. So let's try to make a list and see how things are.

Languages in which "gameplay" or an equivalent exists:

English
French
Spanish

Languages in which "gameplay" or an equivalent doesn't exist:

Japanese
Greek

I also don't think it exists in French, though I do think they have borrowed the English word. It's been a long time since I read a French magazine, however (and back then I wasn't paying attention to the issue since I was using the word myself), so I'll let a French person enlighten us.

When the list is done I'll post it on the frontpage. The idea is to show that millions of people from across the world are doing perfectly fine discussing games without this word or an equivalent.

[El Chaos]

In Spanish there is "jugabilidad", which actually translates more accurately to "playability", though I've seen it used in a similar fashion to gameplay, like "buena y mala jugabilidad" (good and bad playability), and of course magazines rating games' jugabilidad on a percentile scale.

[icycalm]

In French it's called jouabilité, and again means playability. And yeah, I guess they mean the same thing as gameplay, so I'll add French and Spanish to the list.

[icycalm]

See in Japanese and Greek, since there's no equivalent of the word, when you want to express approval of a game you have to say it's "a good game". I.e. you can't divorce the interactive aspect when delivering a judgement on the entire game.