But is also adds the language tag to the sef string, which is incorrect. The sef string has already the language part in it.
For example, the generated product URL, sef option

http:/http://www.abc.com/nl/volkswagen/vw-passat-variant.html?lang=nl
As you can see there is already /nl/ done by Joomla sef.
The added ?lang=nl to your string should not be there!
BTW if I process with sef off, you get an error
Using VM 3.0.6