Popup calendar/First date of week

Where can I change (or hack/override) first day of week in popup calendars?

As first day of week is language dependent, it would be great to have a setting in CB Global config or included in the language package.
(I may have missed something here)

It's a language string. Specifically UE_CALENDAR_FIRSTDAY. You can use language overrides to quickly adjust it per language plugin. See the below on how to use language overrides.

www.joomlapolis.com/blog/kyle/18701-language-overrides-have-arrived

It expects a value of 0 through 6 as per jQuery Datepicker API below.

api.jqueryui.com/datepicker/#option-firstDay

Yes, it did work. Thanks!
UE_CALENDAR_FIRSTDAY should be set to 1 for Monday first.

However, isn´t this string present in a language file?
I did a search on Transifex, in the language folder structure and in the language plugin, but no search hit.
I would have added this for sv-SE (Swedish) but it wasn´t possible.

However, isn´t this string present in a language file?

Nope, has to be manually added as it's not defined anywhere initially.

Would you accept this as a feature request?

As a feature to what? The feature already exists. By default it's not supposed to be used and uses whatever jQuery UI has been defined with. That usage overrides jQuery UI.