Datepicker language not everywhere the same language

I noticed that the Date Picker (flatpickr) did not take the language dutch everywhere. This can be seen on page: https://buurtgerichtezorg.com/voeg-project-toe/ at “Periode van uitvoering” “to” is used instead of “tot” in dutch and on the detail page: https://buurtgerichtezorg.com/project/sint-pietersvliet-7-2000-antwerpen-finaal-test-project-dennis/ “Periode van uitvoering” this is completely in English and should therefore be completely in dutch. Can this problem be solved since the datepicker itself can be in Dutch and selected?

The page I need help with: [log in to see the link]