Currently Firefox has two language settings. One for the Firefox app/UI language and one for the preferred language of page content.
The <input type="datetime-local"> element shows a localized input field for entering dates/times. However, the format chosen is based on the setting for the Firefox UI language. So, in my case being european but using English as the Firefox language I get US-formatted date controls, and not the date formatting I'm used to.
Basing the date formatting in these inputs on either the set system locale for date/time format (best) or the setting for preferred page content language (better) would in my mind make more sense and fix these issues for dual-language users like me who prefer English as a UI language but still view local sites in our own language and prefer date formatting based on where we live.