There are multiple programming languages that can be used, as evidenced by the wide variety of apps listed on https://LINMOBapps.frama.io

Besides Python there’s C, C++, Java, Go, Rust and and ... anything that has toolkit bindings basically. I plan to add the main programming languages per app in a future revision of the list.

Other languages can work too. Some tutorials: https://developer.puri.sm/Librem5/Apps/index.html.

Its the same as developing for a linux desktop. mmsd is written in C, and has python unit tests.

Librem runs Progressive Web Apps.