I can. I've developed and released an app for SailfishOS, and am currently in the process of developing a second one.
I do all my app development in emacs, and VirtualBox (running an i486 version of SailfishOS). My app's written in Python. The UI parts are written in QML (basically JS). I have a Makefile that does an assortment of things for me (setup dev env, package application, clean up stuff).
My second app is going to be written in Python and Go, with the UI once again in QML.
It's real Linux underneath, so you can write your application any way you like (as long as it runs on Linux/arm). However, it's easiest to write the UI in QML, as it handles all the input/widgets for you.