Hi @erlend_sh,
I noticed that Monday is already the start day for Pikaday (library used for calendar widgets). Could you tell me in which part of Discourse have you encountered Sunday as a first day?
Anyway, I started working on this and already submitted a PR for it here.
Currently, you can actually choose any day of the week as a first day, which is silly, but I will change that as soon as I am sure that all relevant places have been touched.