The naming convention for translation files:

  pickadate.LANGUAGE_COUNTRY.js

Where:

  LANGUAGE = The lowercase ISO 639-1 language code. See http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
  COUNTRY = The uppercase ISO 3166-1 country code. See http://en.wikipedia.org/wiki/ISO_3166-1_alpha-2#Officially_assigned_code_elements