Steps to reproduce:

1. Use French language as Back Office's one - https://doc.ibexa.co/en/3.3/guide/back_office_translations/
2. Set the language to French in User Settings.
3. Add Date fieldtype to Folder Content Type.
4. Create Folder and try to set a date in the mentioned fieldtype.

Result:
The flatpickr widget instance is still in English language.

Expected result:
The flatpickr widget language should be French.

Note:
I18n widget files are present in the assets directory so this is a matter of providing the proper language to locale property in flatpickr.