I have developed a local plugin which contains a form.
In this form I have a date_selector element.
My problem is that days are translated (in french) but months are still in english.
After some research, I found that I have the same issue when I try to change the date of start of a course. The date picker of the course parameters page has days in french but months always in english.

Steps to reproduce (works e.g. on https://sandbox.moodledemo.net, too):

• Prerequisite: a language pack is installed, e.g. the french one
• Log in as admin
• change the user language to french (the site default language is still English)
• browse to a course page and click on "Edit settings" in the Administration block (opening the course's settings page)
• click on date picker icon next to "Course start date"
• you will see the month name in English instead of current language