pat-datetime-picker relies on browser language configuration instead of Plone configuration

I don't know if I should open this here or in Patternslib...

The pat-datetime-picker pattern used in Plone 6 relies on browser interface language to render itself, producing different output in at least Firefox and Chrome.

This is Firefox configured to have the interface in basque:

This is Chrome configured to have the interface in english (I can't set basque as interface language)

As you can see, not only the date picker shows in English, but the datetime format of the input box is also formatted in English.

It looks like the pat-datetime-picker degrades to HTML5 date picker which relies on the browser language.

I would say this is a bug. The date picker should be rendered not in the browser language but in the content language. I don't know if this can be set in the HTML5 standard date-picker (I don't see how here, so perhaps we should render the JS date-picker anyway?

cc: @libargutxi

Start by tracing the pat-datetime-picker pattern and how it falls back to the HTML5 date picker in Plone 6. Compare the browser-dependent rendering described for Firefox and Chrome, then determine how the content language should control the picker and input format. Done means the date picker and datetime format follow Plone configuration consistently across browsers.