One of the properties a document can have is called Category.
In Word 2007, you can access it this way: Microsoft Office button > point to Prepare and choose Properties. Type the word English in the Category box near the top of the window.

Of course, if ithere would be fewer docs to edit, you reverse the code a bit...

{IF {DOCPROPERTY Category}=”NL” {STYLEREF “Kop 1”} {STYLEREF “Heading 1”}}

In this case, if will use Kop 1 if you have NL in the Category property, otherwise it will use Heading 1.