You can write programs using Bela's signature ultra low-latency real-time audio and sensor I/O using:
- C++
- PureData
- Supercollider
- Csound
- FAUST
So Javascript and Python are not supported, because they are garbage collected (among other things) and so are not real time friendly. This said, at some point we had support for pyo "a Python module containing classes for a wide variety of audio signal processing types": https://pypi.org/project/pyo/ . It shouldn't take much to make it work again, if there is interest.
As for the operating system, the "stable" image comes with Debian Stretch and there is an experimental Debian Bullseye version (which is actually pretty good and I use it daily). Besides some tweaking, the main difference between a Bela image and a stock Debian image is the presence of the Bela files and a kernel containing the Xenomai patches.
This said, if you don't care for the ultra low-latency real-time audio and sensor I/O, then you can just use any distro/kernel/programming language, load the appropriate device overlay for the Bela cape's audio codec to show up as an ALSA soundcard and do just any good old regular Linux stuff with it.