How can I translate the captions to the date field to another language?

the date field doesn't show up among the fields to translate?

I have checked the field element and indeed you are not able to translate the date field question.

This is because you are using "BirthDate Widget " for your date questions and one can not translate the Widget texts bit you can only translate the widget label only.

Also available is a Date-Picker element which you can use instead of the Birthdate widget and limit to day, month and year in your form. The advantage you can translate the date-picker element.

Below is an illustration of the date-picker element which you can be able to run translation on.