Thanks for your response, but it seems more complicated than that. I didn’t answer for some days because I am digging in the problem before overwhelming you with vague descriptions (and because I had some other priorities as well).
Well, my time and language settings are OK screenshot
I found something that may be related to the problem. With scheduled alerts, I see a problem with the timepicker when scheduling an alert.
Using Firefox: screenshot
Using Chromium: screenshot
The buggy arrows on the screenshot means that if you click them the time is initialized and becomes jj/mm/aaaa
In both cases, the adminbar tab shows months in english: screenshot
All javascripts of the browsers are enabled. I will come back if I find something else that can help.
Have a nice day!