Modifying Numbers calendar template - adding holidays & custom text

A couple unresolved issues came up about modifying a calendar using the calendar template available in Numbers. I am moderately fluent in Numbers so bear with me...

I'd like to insert holidays with their names into each monthly display. I am using holidays from several countries so I imagine easiest is populating list from Google or calendar service and converting .ics to .csv for Numbers. I'm building a list for 2021 currently, but would be for future years as well

-How would I create an holiday list to show the names on specific dates?

Secondly, I'd like to change the language of the months. I can change the days of the week in the top grid bar with language&country option, however I do not see a way to change the month text in the pop-up menu. Screenshot below