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.