What programming language can I use to develop a Retailer?
You can use almost all programming languages, the only requirement is this programming language can create an HTTP server and this HTTP server need to implement Retailer RESTFul API. So you can use Java, Python, NodeJS, Golang, and so on.
If you don't have a preferred language, then I suggest you use NodeJS:
BitSky Desktop has an integrated Retailer Editor for NodeJS
What database supported by BitSky?
Technical BitSky should be able to support all the databases that TypeORM supports, but currently, we only tested with SQLite and MongoDB, so for now, we only support SQLite and MongoDB. Since BitSky is open-source software, we have limit resources, please create a feature ticket to let us know what you need.