Convert international datestring to ISO-format
Neat stuff. 

Suggestions:

Changing shortmonths to a dictionary.  This will allow for a pair between numerical months and alphabetical months.  No need to repeat 'jan' for example, as you have it now.

Pythonic: unpack month, year, day in a one liner. 

Use datetime's strftime to format dates...makes life easier in case you want to change the format down the road.

import re
import datetime

def ISOdate(date):

    month_d = {'01': 'jan',
               '02': 'feb',
               '03': ['mar', 'mrt'],
               '04': 'apr',
               '05': ['may', 'mei'],
               '06': 'jun',
               '07': 'jul',
               '08': 'aug',
               '09': 'sep',
               '10': ['oct', 'okt'],
               '11': 'nov',
               '12': 'dec'
               }

     pat = r'(\d{1,2})\s?[-\/]?\s?(\w{3})\s?[-\/]?\s?(\d{4})'
     q = re.match(pat, date)

     if q:
         day, month, year = [q.group(idx+1) for idx in range(3)]
         if month.isalpha(): # change from letters to numbers
             month = [k for k, v in month_d.items() if month in v][0]
         out_date = datetime.date(int(year), int(month), int(day))
         return datetime.datetime.strftime(out_date, '%Y-%m-%d')  

     else:
         return date