netspinlab hi,
The full-calendar plugin is left untouched. We did not change anything to the plugin so I am not sure why it wouldn't work. Only thing I can do is post a question on their forum page and basically just ask why it isn't working. Maybe you missed a settings somewhere.
Also, I wanted to confirm, I have gone through all our codes for full-calendar and there is nothing we hard-coded...
...
let calendar = new Calendar(calendarEl, {
locales: [ esLocale, frLocale ],
locale: 'fr' // the initial locale. of not specified, uses the first one
});
...
calendar.setOption('locale', 'es');