The German date format is 01.05.2015 as opposed to the UK date format 01/05/2015 which Tablesorter.js comes prebuilt to parse.

To sort by a German date, you have to add a custom parser. Here is one that will work for a date with the German dot notation.

  id: 'dotDate',
  is: function (content) {
    return /\d{1,2}\.\d{1,2}\.\d{1,4}/.test(content);
  format: function (content) {
    content = content.replace(/\-/|:|\.|\//g,' ').split(' ');
    return $.tablesorter.formatFloat(new Date(content[2], content[1]-1, content[0]).getTime());
  type: 'numeric'

Simply add this parser and then tell your tablesorter initializer function which column the German date is in.

  headers: {
    0: { sorter: 'dotDate' }

If your date only has 2 numbers for the year instead of 4, you can adjust the is method to use the regex /\d{1,2}\.\d{1,2}\.\d{1,2}/.