How to get the name of the month in local language in Drupal?
Don't use the PHP date function, use the Drupal API: format_date. To add the comma, make it part of the pattern. To match your example 30 maart 2015, 11: 34, the pattern is j F Y, H: i:

$timestamp = strtotime('2015-03-30 11:34');
print format_date($timestamp, 'custom', 'j F Y, H: i', NULL, 'nl');


For this to work, you need to have the core locale module enabled and Dutch translations on your site should be up to date (check that the names of months are translated inthe translation UI).  The module Localization Update will help you with updating translations if you need to.

You can skip the last argument ('nl') if the default language of your site is dutch.