I think in Developer's guide on http://www.contao.org/reference.html page there is a missing key in "Fields" section.
Missing key is "value". So it should be:
Key: value
Value: &$GLOBALS['TL_LANG'] string
Description: Default value of current filed. Typically a reference to the global language array. (text and textarea fields only).
Maybe I'm wrong but I use this key in fields in my DCA and it's works.
