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.

$.tablesorter.addParser({
  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.

$('table').tablesorter({
  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}/.