You can use the IF field to have Word determine which to use.
Use other fields to create the expression. I used the document property 'Category':

If you put the word English in the Category property of the doc, then the STYLEREF will be Heading 1, otherwise Kop 1:

You cannot type in the curly braces. Type Ctrl+F9 to create them.

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

You can use something other than a doc property, hopefully this will give you some ideas.