We started with Lua, but just couldn’t hack the syntax. Python would have been nice but with both Python and JS, there were no VMs we could find that would work with the resources the imp has.
There is no subscription for developers, this is free – but yes, commercial use (ie making and resale of imp-based devices intended for non-developers) requires subscription.
As of release23, you can run local code without a network connection, and connect/disconnect under program control.