These parameters %-d %B generate results according to the operating system’s locale setting.
On the machine running Home Assistant, what is its default locale setting? I assume your local language is Swedish so is the machine’s locale set to Swedish? I think the language code is sv_SE.UTF-8.
Unfortunately, I am unfamiliar with how locale works with Hass.io. I believe you select the desired language during its installation. I don’t know if it can be changed afterwards. Was your Hass.io installed using Swedish as the chosen language?
Yes. I have been checking and I have not found an way to resolve it in Hassio. Will continue to look around. Worst case I will have to use the workaround proposed above.
No - I still use the workaround. In most instances hassio returns Swedish responses for me. But in some cases like the one above I do not get an English response. No idea why.