I made the code a little better:
\startluacode
  os.setlocale("nl_NL")

  function modification_date(file, format)
    local f    = file
    local attr = lfs.attributes(f)
    local fmt  = "%d %B %Y"

    assert (type(attr) == "table")
    if attr.mode ~= "directory" then
      if format == "long" then
        fmt = fmt .. " %H:%M"
      end
      return os.date(fmt,attr.modification)
    end
  end
\stopluacode

There is only one problem. I get:
    25 March 2011
instead of:
    25 maart 2011

What am I doing wrong?