The Mac App Store icon infers that it's a Mac app. As for languages, you're right that it isn't clear. Having said that, the main window is a text editor which performs highlighting on all of the major web languages (in Coda 1.x anyway).
If you're not a Mac user, how would you know what the App Store icon looks like?
There was nothing in the Help/FAQ either, so it wasn't until I manually navigated to panic.com, seeing that they do Apple apps, that I finally got pretty sure it wouldn't be available for my platform.
There comes a point where preparing for all levels of pedantry isn't wise and it's fine for people to assume a certain level of knowledge/experience from their target audience especially if their target audience isn't everyone.
"You code the web. We revolutionized that process in Coda, putting everything in one place. An editor. Terminal. CSS. Files."