Hi Thomas,
In the current versions of Questionnaire (for Moodle 2) the "output responses as text" works OK for dates, irrespective of the language being used.
Since you insisted I did carry out the dump test you suggested on the response_select() function for date questions. I did notice a discrepancy between the number of elements in the date questions as opposed to the other question types. This comes from the insertion of 2 empty elements in this line (in the response_date processing).
array_push($values["$qid"], '', '', $val);
Thanks for pointing to this inconsistency, which does not have any impact on the output of responses, but I will fix it ASAP.
Joseph
PS.- Do you agree with this "Well the dd/mm/yyyy date format is not "bizarre", it is the standard format for the Dutch language (and also for French)." ?