Depends what you mean by decentralised app, ie what you want it to do, and what languages you prefer. You can write web apps now, also desktop apps easily using nodejs. It is feasible to use other languages, but not so easily because the language bindings to SAFE API are limited just now, but being expanded I believe. All these options rely on the SAFE browser for account creation and authentication.
If you want to do away with the browser for authentication, and authenticate yourself, things are significantly more complicated, but you can do this. You will though have to dig deeper into the network protocols or at least the rust libraries themselves rather than the APIs published so far because those rely on browser authenticator.
I think that’s a fair summary.